feat(core): support ai network search (#9357)

### What Changed?
- Add `PerplexityProvider` in backend.
- Update session prompt name if user toggle network search mode in chat panel.
- Add experimental flag for AI network search feature.
- Add unit tests and e2e tests.

Search results are streamed and appear word for word:

<div class='graphite__hidden'>
          <div>🎥 Video uploaded on Graphite:</div>
            <a href="https://app.graphite.dev/media/video/sJGviKxfE3Ap685cl5bj/56f6ec7b-4b21-405f-9612-43e083f6fb84.mov">
              <img src="https://app.graphite.dev/api/v1/graphite/video/thumbnail/sJGviKxfE3Ap685cl5bj/56f6ec7b-4b21-405f-9612-43e083f6fb84.mov">
            </a>
          </div>
<video src="https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/sJGviKxfE3Ap685cl5bj/56f6ec7b-4b21-405f-9612-43e083f6fb84.mov">录屏2024-12-27 18.58.40.mov</video>

Click the little globe icon to manually turn on/off Internet search:
<div class='graphite__hidden'>
          <div>🎥 Video uploaded on Graphite:</div>
            <a href="https://app.graphite.dev/media/video/sJGviKxfE3Ap685cl5bj/778f1406-bf29-498e-a90d-7dad813392d1.mov">
              <img src="https://app.graphite.dev/api/v1/graphite/video/thumbnail/sJGviKxfE3Ap685cl5bj/778f1406-bf29-498e-a90d-7dad813392d1.mov">
            </a>
          </div>
<video src="https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/sJGviKxfE3Ap685cl5bj/778f1406-bf29-498e-a90d-7dad813392d1.mov">录屏2024-12-27 19.01.16.mov</video>

When there is an image, it will automatically switch to the openai model:

<div class='graphite__hidden'>
          <div>🎥 Video uploaded on Graphite:</div>
            <a href="https://app.graphite.dev/media/video/sJGviKxfE3Ap685cl5bj/56431d8e-75e1-4d84-ab4a-b6636042cc6a.mov">
              <img src="https://app.graphite.dev/api/v1/graphite/video/thumbnail/sJGviKxfE3Ap685cl5bj/56431d8e-75e1-4d84-ab4a-b6636042cc6a.mov">
            </a>
          </div>
<video src="https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/sJGviKxfE3Ap685cl5bj/56431d8e-75e1-4d84-ab4a-b6636042cc6a.mov">录屏2024-12-27 19.02.13.mov</video>

packages/backend/server/src/plugins/copilot/resolver.ts

@@ -22,6 +22,7 @@ import GraphQLUpload from 'graphql-upload/GraphQLUpload.mjs';
 import {
   CallMetric,
   CopilotFailedToCreateMessage,
+  CopilotSessionNotFound,
   FileUpload,
   RequestMutex,
   Throttle,
@@ -62,6 +63,17 @@ class CreateChatSessionInput {
   promptName!: string;
 }
 
+@InputType()
+class UpdateChatSessionInput {
+  @Field(() => String)
+  sessionId!: string;
+
+  @Field(() => String, {
+    description: 'The prompt name to use for the session',
+  })
+  promptName!: string;
+}
+
 @InputType()
 class ForkChatSessionInput {
   @Field(() => String)
@@ -372,6 +384,38 @@ export class CopilotResolver {
     });
   }
 
+  @Mutation(() => String, {
+    description: 'Update a chat session',
+  })
+  @CallMetric('ai', 'chat_session_update')
+  async updateCopilotSession(
+    @CurrentUser() user: CurrentUser,
+    @Args({ name: 'options', type: () => UpdateChatSessionInput })
+    options: UpdateChatSessionInput
+  ) {
+    const session = await this.chatSession.get(options.sessionId);
+    if (!session) {
+      throw new CopilotSessionNotFound();
+    }
+    const { workspaceId, docId } = session.config;
+    await this.permissions.checkCloudPagePermission(
+      workspaceId,
+      docId,
+      user.id
+    );
+    const lockFlag = `${COPILOT_LOCKER}:session:${user.id}:${workspaceId}`;
+    await using lock = await this.mutex.acquire(lockFlag);
+    if (!lock) {
+      return new TooManyRequest('Server is busy');
+    }
+
+    await this.chatSession.checkQuota(user.id);
+    return await this.chatSession.updateSessionPrompt({
+      ...options,
+      userId: user.id,
+    });
+  }
+
   @Mutation(() => String, {
     description: 'Create a chat session',
   })