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:

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          <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:
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          <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:

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          <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/tests/utils/copilot.ts

@@ -184,6 +184,31 @@ export async function createCopilotSession(
   return res.body.data.createCopilotSession;
 }
 
+export async function updateCopilotSession(
+  app: INestApplication,
+  userToken: string,
+  sessionId: string,
+  promptName: string
+): Promise<string> {
+  const res = await request(app.getHttpServer())
+    .post(gql)
+    .auth(userToken, { type: 'bearer' })
+    .set({ 'x-request-id': 'test', 'x-operation-name': 'test' })
+    .send({
+      query: `
+        mutation updateCopilotSession($options: UpdateChatSessionInput!) {
+          updateCopilotSession(options: $options)
+        }
+      `,
+      variables: { options: { sessionId, promptName } },
+    })
+    .expect(200);
+
+  handleGraphQLError(res);
+
+  return res.body.data.updateCopilotSession;
+}
+
 export async function forkCopilotSession(
   app: INestApplication,
   userToken: string,