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/frontend/core/src/modules/ai-button/index.ts

@@ -3,11 +3,21 @@ export { AIButtonService } from './services/ai-button';
 
 import type { Framework } from '@toeverything/infra';
 
+import { FeatureFlagService } from '../feature-flag';
+import { GlobalStateService } from '../storage';
 import { AIButtonProvider } from './provider/ai-button';
 import { AIButtonService } from './services/ai-button';
+import { AINetworkSearchService } from './services/network-search';
 
 export const configureAIButtonModule = (framework: Framework) => {
   framework.service(AIButtonService, container => {
     return new AIButtonService(container.getOptional(AIButtonProvider));
   });
 };
+
+export function configureAINetworkSearchModule(framework: Framework) {
+  framework.service(AINetworkSearchService, [
+    GlobalStateService,
+    FeatureFlagService,
+  ]);
+}

packages/frontend/core/src/modules/ai-button/services/network-search.ts

@@ -0,0 +1,45 @@
+import {
+  createSignalFromObservable,
+  type Signal,
+} from '@blocksuite/affine-shared/utils';
+import { LiveData, Service } from '@toeverything/infra';
+
+import type { FeatureFlagService } from '../../feature-flag';
+import type { GlobalStateService } from '../../storage';
+
+const AI_NETWORK_SEARCH_KEY = 'AINetworkSearch';
+
+export class AINetworkSearchService extends Service {
+  constructor(
+    private readonly globalStateService: GlobalStateService,
+    private readonly featureFlagService: FeatureFlagService
+  ) {
+    super();
+
+    const { signal: enabled, cleanup: enabledCleanup } =
+      createSignalFromObservable<boolean | undefined>(this._enabled$, false);
+    this.enabled = enabled;
+    this.disposables.push(enabledCleanup);
+
+    const { signal: visible, cleanup: visibleCleanup } =
+      createSignalFromObservable<boolean | undefined>(this._visible$, false);
+    this.visible = visible;
+    this.disposables.push(visibleCleanup);
+  }
+
+  visible: Signal<boolean | undefined>;
+
+  enabled: Signal<boolean | undefined>;
+
+  private readonly _visible$ =
+    this.featureFlagService.flags.enable_ai_network_search.$;
+
+  private readonly _enabled$ = LiveData.from(
+    this.globalStateService.globalState.watch<boolean>(AI_NETWORK_SEARCH_KEY),
+    false
+  );
+
+  setEnabled = (enabled: boolean) => {
+    this.globalStateService.globalState.set(AI_NETWORK_SEARCH_KEY, enabled);
+  };
+}