feat: add doc copilot context api (#10103)

### What Changed?
- Add graphql APIs.
- Provide context and session service in `AIProvider`.
- Rename the state from `embedding` to `processing`.
- Reafctor front-end session create, update and save logic.

Persist the document selected by the user:
[录屏2025-02-08 11.04.40.mov <span class="graphite__hidden">(uploaded via Graphite)</span> <img class="graphite__hidden" src="https://app.graphite.dev/api/v1/graphite/video/thumbnail/sJGviKxfE3Ap685cl5bj/195a85f2-43c4-4e49-88d9-6b5fc4f235ca.mov" />](https://app.graphite.dev/media/video/sJGviKxfE3Ap685cl5bj/195a85f2-43c4-4e49-88d9-6b5fc4f235ca.mov)

packages/backend/server/src/plugins/copilot/context/resolver.ts

@@ -36,12 +36,12 @@ class AddContextDocInput {
 }
 
 @InputType()
-class RemoveContextFileInput {
+class RemoveContextDocInput {
   @Field(() => String)
   contextId!: string;
 
   @Field(() => String)
-  fileId!: string;
+  docId!: string;
 }
 
 @ObjectType('CopilotContext')
@@ -227,8 +227,8 @@ export class CopilotContextResolver {
   })
   @CallMetric('ai', 'context_doc_remove')
   async removeContextDoc(
-    @Args({ name: 'options', type: () => RemoveContextFileInput })
-    options: RemoveContextFileInput
+    @Args({ name: 'options', type: () => RemoveContextDocInput })
+    options: RemoveContextDocInput
   ) {
     const lockFlag = `${COPILOT_LOCKER}:context:${options.contextId}`;
     await using lock = await this.mutex.acquire(lockFlag);
@@ -238,7 +238,7 @@ export class CopilotContextResolver {
     const session = await this.context.get(options.contextId);
 
     try {
-      return await session.removeDocRecord(options.fileId);
+      return await session.removeDocRecord(options.docId);
     } catch (e: any) {
       throw new CopilotFailedToModifyContext({
         contextId: options.contextId,