feat(server): gemini embedding 2 support (#14956)

#### PR Dependency Tree


* **PR #14956** 👈

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## Summary by CodeRabbit

## Release Notes

* **Bug Fixes**
* Improved Gemini Vertex provider configuration validation logic for
enhanced reliability.
  * Refined Google Vertex publisher base URL construction handling.

* **Tests**
  * Added test coverage for Gemini Embedding 2 model resolution.
* Added test coverage for Gemini Vertex provider Google Cloud
integration.

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packages/backend/server/src/__tests__/copilot/native-provider.spec.ts

@@ -25,6 +25,7 @@ import {
   type PromptMessage,
   type StreamObject,
 } from '../../plugins/copilot/providers/types';
+import { getVertexGoogleBaseUrl } from '../../plugins/copilot/providers/utils';
 import {
   buildPromptStructuredResponseFromFields,
   buildStructuredResponseContract,
@@ -1823,6 +1824,17 @@ test('GeminiVertexProvider should prefetch bearer token for native config', asyn
   t.snapshot(config);
 });
 
+test('GeminiVertexProvider should build project scoped Vertex base URL', t => {
+  t.is(
+    getVertexGoogleBaseUrl({
+      project: 'p1',
+      location: 'us-central1',
+      googleAuthOptions: {},
+    }),
+    'https://us-central1-aiplatform.googleapis.com/v1/projects/p1/locations/us-central1/publishers/google'
+  );
+});
+
 test('GeminiVertexProvider should materialize remote attachments before native text path', async t => {
   const cases = [
     {