feat(core): add ai file context api (#10842)

Close [BS-2349](https://linear.app/affine-design/issue/BS-2349).

### What Changed?
- Add file context graphql apis
- Pass matched file chunks to LLM

[录屏2025-02-19 23.27.47.mov <span class="graphite__hidden">(uploaded via Graphite)</span> <img class="graphite__hidden" src="https://app.graphite.dev/api/v1/graphite/video/thumbnail/sJGviKxfE3Ap685cl5bj/8e8a98ca-6959-4bb6-9759-b51d97cede49.mov" />](https://app.graphite.dev/media/video/sJGviKxfE3Ap685cl5bj/8e8a98ca-6959-4bb6-9759-b51d97cede49.mov)

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

@@ -141,6 +141,7 @@ export class ChatSession implements AsyncDisposable {
   finish(params: PromptParams): PromptMessage[] {
     const messages = this.takeMessages();
     const firstMessage = messages.at(0);
+    // TODO: refactor this {{content}} keyword agreement
     // if the message in prompt config contains {{content}},
     // we should combine it with the user message in the prompt
     if (