fix: recursive flattening for nested error unions (#347)

## Why

Fixed a bug in the `flattenErrorType` utility where spreading a
flattened union into another union was not working as expected.

## What changed

- Fixed the `Flatten` type in `router/errors.ts` to properly handle
nested unions (again)
- Added a test case that demonstrates spreading a flattened union into
another union
- Bumped package version from 0.210.0 to 0.210.1

## Versioning

- [ ] Breaking protocol change
- [ ] Breaking ts/js API change

---------

Co-authored-by: Jacky Zhao <j.zhao2k19@gmail.com>

Author: darshkpatel

__tests__/typescript-stress.test.ts

@@ -689,6 +689,32 @@ describe('Procedure error schema', () => {
         ),
       );
     });
+
+    test('spreading a flattened union into another union', () => {
+      const BaseErrorSchema = flattenErrorType(
+        Type.Union([
+          Type.Object({
+            code: Type.Literal('NOT_FOUND'),
+            message: Type.String(),
+          }),
+          Type.Object({
+            code: Type.Literal('PARSE_ERROR'),
+            message: Type.String(),
+          }),
+        ]),
+      );
+
+      // Then we want to combine it with another error in a new union
+      const ExtendedErrorSchema = Type.Union([
+        Type.Object({
+          code: Type.Literal('NEW_ERROR'),
+          message: Type.String(),
+        }),
+        BaseErrorSchema,
+      ]);
+
+      acceptErrorSchema(ExtendedErrorSchema);
+    });
   });
 
   describe('fails', () => {

package-lock.json

@@ -1,7 +1,7 @@
 {
   "name": "@replit/river",
   "description": "It's like tRPC but... with JSON Schema Support, duplex streaming and support for service multiplexing. Transport agnostic!",
-  "version": "0.210.0",
+  "version": "0.210.1",
   "type": "module",
   "exports": {
     ".": {

package.json

@@ -139,12 +139,10 @@ function isUnion(schema: TSchema): schema is TUnion {
   return schema[Kind] === 'Union';
 }
 
-type Flatten<T> = T extends BaseErrorSchemaType
+export type Flatten<T> = T extends BaseErrorSchemaType
   ? T
   : T extends TUnion<Array<infer U extends TSchema>>
-  ? U extends BaseErrorSchemaType
-    ? TUnion<Array<U>>
-    : Flatten<U>
+  ? Flatten<U>
   : unknown;
 
 /**