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>

yarn.lock

@@ -808,6 +808,7 @@ __metadata:
     cookie-parser: "npm:^1.4.7"
     cross-env: "npm:^7.0.3"
     dotenv: "npm:^16.4.7"
+    eventsource-parser: "npm:^3.0.0"
     express: "npm:^4.21.2"
     fast-xml-parser: "npm:^4.5.0"
     get-stream: "npm:^9.0.1"
@@ -21165,6 +21166,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"eventsource-parser@npm:^3.0.0":
+  version: 3.0.0
+  resolution: "eventsource-parser@npm:3.0.0"
+  checksum: 10/8215adf5d8404105ecd0658030b0407e06987ceb9aadcea28a38d69bacf02e5d0fc8bba5fa7c3954552c89509c8ef5e1fa3895e000c061411c055b4bbc26f4b0
+  languageName: node
+  linkType: hard
+
 "execa@npm:^1.0.0":
   version: 1.0.0
   resolution: "execa@npm:1.0.0"