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/frontend/graphql/src/schema.ts

@@ -2355,6 +2355,7 @@ export type AddContextFileMutation = {
     createdAt: number;
     name: string;
     chunkSize: number;
+    error: string | null;
     status: ContextFileStatus;
     blobId: string;
   };
@@ -2385,6 +2386,7 @@ export type ListContextFilesQuery = {
           name: string;
           blobId: string;
           chunkSize: number;
+          error: string | null;
           status: ContextFileStatus;
           createdAt: number;
         }>;