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/mobile/dialogs/setting/experimental/index.tsx

@@ -46,6 +46,7 @@ const ExperimentalFeatureList = () => {
       {Object.keys(AFFINE_FLAGS).map(key => (
         <ExperimentalFeaturesItem
           key={key}
+          flagKey={key}
           flag={featureFlagService.flags[key as keyof AFFINE_FLAGS]}
         />
       ))}
@@ -53,7 +54,13 @@ const ExperimentalFeatureList = () => {
   );
 };
 
-const ExperimentalFeaturesItem = ({ flag }: { flag: Flag }) => {
+const ExperimentalFeaturesItem = ({
+  flag,
+  flagKey,
+}: {
+  flag: Flag;
+  flagKey: string;
+}) => {
   const t = useI18n();
   const value = useLiveData(flag.$);
 
@@ -72,7 +79,7 @@ const ExperimentalFeaturesItem = ({ flag }: { flag: Flag }) => {
     <li>
       <div className={styles.itemBlock}>
         {t[flag.displayName]()}
-        <Switch checked={value} onChange={onChange} />
+        <Switch data-testid={flagKey} checked={value} onChange={onChange} />
       </div>
       {flag.description ? (
         <div className={styles.itemDescription}>{t[flag.description]()}</div>