[ES|QL] fix: Selecting a lookup index alias adds the $0 suffix (#234026)

Closes #233260

## Summary
Fixes a wrongly built completion item that was rendering a snippet
symbol when selecting an index alias.

Added a validation check in the test completion helper that verifies
that every completion text using a snippet symbol has a `asSnippet`
flag.



https://github.com/user-attachments/assets/b6d8f171-d938-45dc-bcf9-05e0f88a167a


### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

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guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
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Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
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### Identify risks

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Describe the risk, its severity, and mitigation for each identified
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- [ ] ...

Author: sddonne

src/platform/packages/shared/kbn-esql-ast/src/__tests__/autocomplete.ts

@@ -91,6 +91,11 @@ export const expectSuggestions = async (
 
   const suggestions: string[] = [];
   result.forEach((suggestion) => {
+    if (containsSnippet(suggestion.text) && !suggestion.asSnippet) {
+      throw new Error(
+        `Suggestion with snippet placeholder must be marked as a snippet. -> ${suggestion.text}`
+      );
+    }
     suggestions.push(suggestion.text);
   });
   expect(uniq(suggestions).sort()).toEqual(uniq(expectedSuggestions).sort());
@@ -269,3 +274,10 @@ export function getLiteralsByType(_type: SupportedDataType | SupportedDataType[]
   }
   return [];
 }
+
+export const containsSnippet = (text: string): boolean => {
+  // Matches most common monaco snippets
+  // $0, $1, etc. and ${1:placeholder}, ${2:another}
+  const snippetRegex = /\$(\d+|\{\d+:[^}]*\})/;
+  return snippetRegex.test(text);
+};

src/platform/packages/shared/kbn-esql-ast/src/commands_registry/commands/join/autocomplete.ts

@@ -54,6 +54,7 @@ export async function autocomplete(
           ({
             label: mnemonic,
             text: mnemonic + ' $0',
+            asSnippet: true,
             detail: description,
             kind: 'Keyword',
             sortText: `${i}-MNEMONIC`,

src/platform/packages/shared/kbn-esql-ast/src/definitions/utils/sources.ts

@@ -205,6 +205,7 @@ export const specialIndicesToSuggestions = (
         aliasSuggestions.push({
           label: alias,
           text: alias + ' $0',
+          asSnippet: true,
           kind: 'Issue',
           detail: i18n.translate('kbn-esql-ast.esql.autocomplete.specialIndexes.indexType.alias', {
             defaultMessage: 'Alias',