[dashboards as code] links embeddable schema (elastic#239354)

## Summary

Fixes elastic#238089

Registers links embeddable schema. 


### Testing OAS documentation
* add `server.oas.enabled: true` to config/kibana.dev.yml
* Start kibana with `yarn start --no-base-path`
* Copy paste URL
`http://localhost:5601/api/oas?pathStartsWith=/api/dashboard&access=internal&version=1`
into your browser.
* View `panels` schema and verify links embeddable schema is in oneOf
list.

<img width="1065" height="833" alt="Screenshot 2025-10-16 at 15 56 27"
src="https://github.com/user-attachments/assets/ac648c8e-efcb-413f-9084-ad7991e9152f"
/>

### Manual Testing
1. Save an existing dashboard with links panels (both by-value and
by-reference) through the UI - should succeed
2. Clone a dashboard with links panels - should maintain panel
configurations correctly
3. Export and import a dashboard with links panels - should preserve all
link configurations

### Testing Schema Validation

#### Test 1: By-Value Links Panel with Invalid Property (Should Return
400)

Expected Result: Returns 400 Bad Request due to unexpected `invalidKey`
property

```
  POST kbn:/api/dashboards/dashboard?apiVersion=1
  {
    "title": "Links by-value validation - invalid",
    "panels": [
      {
        "config": {
          "links": [
            {
              "label": "My Dashboard Link",
              "type":"dashboardLink","id":"c9c3d1fd-04f9-45ac-89a7-e26e71b785ce",
              "destination":"7adfa750-4c81-11e8-b3d7-01146121b73d",
              "order": 0,
              "options": {
                "openInNewTab": false,
                "useCurrentFilters": true,
                "useCurrentDateRange": true
              }
            }
          ],
          "layout": "vertical",
          "invalidKey": "This should cause a validation error"
        },
        "grid": {
          "x": 0,
          "y": 0,
          "w": 12,
          "h": 8
        },
        "type": "links"
      }
    ]
  }
```

  ---
  Test 2: By-Value Links Panel with Valid Data (Should Succeed)

  Expected Result: Dashboard is created successfully

```
  POST kbn:/api/dashboards/dashboard?apiVersion=1
  {
    "title": "Links by-value validation - Valid",
    "panels": [
      {
        "config": {
          "links": [
            {
              "label": "My Dashboard Link",
              "type":"dashboardLink","id":"c9c3d1fd-04f9-45ac-89a7-e26e71b785ce",
              "destination":"7adfa750-4c81-11e8-b3d7-01146121b73d",
              "order": 0,
              "options": {
                "openInNewTab": false,
                "useCurrentFilters": true,
                "useCurrentDateRange": true
              }
            }
          ],
          "layout": "vertical",
        },
        "grid": {
          "x": 0,
          "y": 0,
          "w": 12,
          "h": 8
        },
        "type": "links"
      }
    ]
  }
```


  ---
Before proceeding, please create a links saved object and save it to the
library. Then replace the `savedObjectId` property in the Post requests
below with the newly created SO ID.

Test 3: By-Reference Links Panel with Invalid Property (Should Return
400)

Expected Result: Returns 400 Bad Request due to unexpected `invalidKey`
property

```
  POST kbn:/api/dashboards/dashboard?apiVersion=1
  {
    "title": "Links by-reference validation - invalid",
    "panels": [
      {
        "config": {
          "savedObjectId": "LINK_SAVED_OBJECT_ID",
          "invalidKey": "This should cause a validation error"
        },
        "grid": {
          "x": 0,
          "y": 0,
          "w": 12,
          "h": 8
        },
        "type": "links"
      }
    ]
  }
```

  ---
  Test 4: By-Reference Links Panel with Valid Data (Should Succeed)

  Expected Result: Dashboard is created successfully

```
  POST kbn:/api/dashboards/dashboard?apiVersion=1
  {
    "title": "Links by-reference validation - valid",
    "panels": [
      {
        "config": {
          "savedObjectId": "LINK_SAVED_OBJECT_ID",
          "title": "My Saved Links",
          "description": "A collection of useful links"
        },
        "grid": {
          "x": 0,
          "y": 0,
          "w": 12,
          "h": 8
        },
        "type": "links"
      }
    ]
  }
```

After creation, navigate to the Dashboard app and verify newly created
dashboards render correctly.

---------

Co-authored-by: kibanamachine <[email protected]>

Author: mbondyra

src/platform/plugins/private/links/common/embeddable/types.ts

@@ -8,14 +8,8 @@
  */
 
 import type { SerializedTitles } from '@kbn/presentation-publishing';
-import type { LinksState, StoredLinksState } from '../../server';
+import type { StoredLinksState } from '../../server';
 
-export interface LinksByReferenceState {
-  savedObjectId: string;
-}
-
-export type LinksByValueState = Pick<LinksState, 'layout' | 'links'>;
-
-export type LinksEmbeddableState = SerializedTitles & (LinksByValueState | LinksByReferenceState);
+export type { LinksByReferenceState, LinksByValueState, LinksEmbeddableState } from '../../server';
 
 export type StoredLinksEmbeddableState = SerializedTitles & StoredLinksState;

src/platform/plugins/private/links/server/content_management/schema/v1/cm_services.ts

@@ -85,23 +85,29 @@ export const externalLinkSchema = schema.object({
   ),
 });
 
+// Shared schema for links array - used by both saved objects and embeddables
+export const linksArraySchema = schema.arrayOf(
+  schema.oneOf([dashboardLinkSchema, externalLinkSchema]),
+  {
+    meta: { description: 'The list of links to display' },
+  }
+);
+
+// Shared schema for layout - used by both saved objects and embeddables
+export const layoutSchema = schema.maybe(
+  schema.oneOf([schema.literal(LINKS_HORIZONTAL_LAYOUT), schema.literal(LINKS_VERTICAL_LAYOUT)], {
+    meta: {
+      description: 'Denote whether to display the links in a horizontal or vertical layout',
+    },
+  })
+);
+
 export const linksSchema = schema.object(
   {
     title: schema.string({ meta: { description: 'A human-readable title' } }),
     description: schema.maybe(schema.string({ meta: { description: 'A short description.' } })),
-    links: schema.arrayOf(schema.oneOf([dashboardLinkSchema, externalLinkSchema]), {
-      meta: { description: 'The list of links to display' },
-    }),
-    layout: schema.maybe(
-      schema.oneOf(
-        [schema.literal(LINKS_HORIZONTAL_LAYOUT), schema.literal(LINKS_VERTICAL_LAYOUT)],
-        {
-          meta: {
-            description: 'Denote whether to display the links in a horizontal or vertical layout',
-          },
-        }
-      )
-    ),
+    links: linksArraySchema,
+    layout: layoutSchema,
   },
   { unknowns: 'forbid' }
 );

src/platform/plugins/private/links/server/embeddable_schemas.ts

@@ -0,0 +1,60 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import type { TypeOf } from '@kbn/config-schema';
+import { schema } from '@kbn/config-schema';
+import { serializedTitlesSchema } from '@kbn/presentation-publishing-schemas';
+import { linksArraySchema, layoutSchema } from './content_management/schema/v1/cm_services';
+
+// Links by-value state schema (contains layout and links)
+const linksByValueStateSchema = schema.object({
+  layout: layoutSchema,
+  links: linksArraySchema,
+});
+
+// Links by-reference state schema (contains savedObjectId)
+const linksByReferenceStateSchema = schema.object({
+  savedObjectId: schema.string({
+    meta: { description: 'The ID of the saved links object' },
+  }),
+});
+
+// Links by-value embeddable schema (by-value state + titles)
+const linksByValueEmbeddableSchema = schema.allOf(
+  [linksByValueStateSchema, serializedTitlesSchema],
+  {
+    meta: {
+      description: 'Links by-value embeddable schema',
+    },
+  }
+);
+
+// Links by-reference embeddable schema (by-reference state + titles)
+const linksByReferenceEmbeddableSchema = schema.allOf(
+  [linksByReferenceStateSchema, serializedTitlesSchema],
+  {
+    meta: {
+      description: 'Links by-reference embeddable schema',
+    },
+  }
+);
+
+// Complete links embeddable schema (union of by-value and by-reference embeddables)
+export const linksEmbeddableSchema = schema.oneOf(
+  [linksByValueEmbeddableSchema, linksByReferenceEmbeddableSchema],
+  {
+    meta: {
+      description: 'Links embeddable schema',
+    },
+  }
+);
+
+export type LinksByValueState = TypeOf<typeof linksByValueStateSchema>;
+export type LinksByReferenceState = TypeOf<typeof linksByReferenceStateSchema>;
+export type LinksEmbeddableState = TypeOf<typeof linksEmbeddableSchema>;

src/platform/plugins/private/links/server/index.ts

@@ -19,6 +19,12 @@ export type {
   StoredLinksState,
 } from './content_management';
 
+export type {
+  LinksByValueState,
+  LinksByReferenceState,
+  LinksEmbeddableState,
+} from './embeddable_schemas';
+
 export const plugin = async (initContext: PluginInitializerContext) => {
   const { LinksServerPlugin } = await import('./plugin');
   return new LinksServerPlugin(initContext);

src/platform/plugins/private/links/server/plugin.ts

@@ -21,6 +21,7 @@ import type { LinksState } from './content_management';
 import { LinksStorage } from './content_management';
 import { linksSavedObjectType } from './saved_objects';
 import { transforms } from '../common/embeddable/transforms/transforms';
+import { linksEmbeddableSchema } from './embeddable_schemas';
 
 export class LinksServerPlugin implements Plugin<object, object> {
   private readonly logger: Logger;
@@ -49,7 +50,10 @@ export class LinksServerPlugin implements Plugin<object, object> {
 
     core.savedObjects.registerType<LinksState>(linksSavedObjectType);
 
-    plugins.embeddable.registerTransforms(LINKS_EMBEDDABLE_TYPE, transforms);
+    plugins.embeddable.registerTransforms(LINKS_EMBEDDABLE_TYPE, {
+      ...transforms,
+      schema: linksEmbeddableSchema,
+    });
 
     return {};
   }

src/platform/plugins/private/links/tsconfig.json

@@ -35,6 +35,7 @@
     "@kbn/visualizations-plugin",
     "@kbn/presentation-containers",
     "@kbn/presentation-publishing",
+    "@kbn/presentation-publishing-schemas",
     "@kbn/react-kibana-context-render",
     "@kbn/share-plugin",
     "@kbn/es-query",