chore(server): bump gemini to ga version (#12845)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Chores**
- Updated Gemini model identifiers from preview versions to their latest
stable versions across prompts and provider configurations. This ensures
continued compatibility and access to the latest Gemini 2.5 Flash and
Pro models. No changes to user-facing features or prompt content.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/providers/gemini/generative.ts

@@ -32,7 +32,7 @@ export class GeminiGenerativeProvider extends GeminiProvider<GeminiGenerativeCon
     },
     {
       name: 'Gemini 2.5 Flash',
-      id: 'gemini-2.5-flash-preview-05-20',
+      id: 'gemini-2.5-flash',
       capabilities: [
         {
           input: [
@@ -46,7 +46,7 @@ export class GeminiGenerativeProvider extends GeminiProvider<GeminiGenerativeCon
     },
     {
       name: 'Gemini 2.5 Pro',
-      id: 'gemini-2.5-pro-preview-06-05',
+      id: 'gemini-2.5-pro',
       capabilities: [
         {
           input: [

packages/backend/server/src/plugins/copilot/providers/gemini/vertex.ts

@@ -15,7 +15,7 @@ export class GeminiVertexProvider extends GeminiProvider<GeminiVertexConfig> {
   readonly models = [
     {
       name: 'Gemini 2.5 Flash',
-      id: 'gemini-2.5-flash-preview-05-20',
+      id: 'gemini-2.5-flash',
       capabilities: [
         {
           input: [
@@ -29,7 +29,7 @@ export class GeminiVertexProvider extends GeminiProvider<GeminiVertexConfig> {
     },
     {
       name: 'Gemini 2.5 Pro',
-      id: 'gemini-2.5-pro-preview-06-05',
+      id: 'gemini-2.5-pro',
       capabilities: [
         {
           input: [