feat(server): add hints for context files (#13444)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Attachments (files) are now included in the conversation context,
allowing users to reference files during chat sessions.
* Added a new "blobRead" tool enabling secure, permission-checked
reading of attachment content in chat sessions.

* **Improvements**
* Enhanced chat session preparation to always include relevant context
files.
* System messages now clearly display attached files and selected
content only when available, improving context clarity for users.
* Updated tool-calling guidelines to ensure user workspace is searched
even when attachment content suffices.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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

@@ -203,6 +203,19 @@ export class ContextSession implements AsyncDisposable {
     return this.config.files.find(f => f.id === fileId);
   }
 
+  async getFileContent(
+    fileId: string,
+    chunk?: number
+  ): Promise<string | undefined> {
+    const file = this.getFile(fileId);
+    if (!file) return undefined;
+    return this.models.copilotContext.getFileContent(
+      this.contextId,
+      fileId,
+      chunk
+    );
+  }
+
   async removeFile(fileId: string): Promise<boolean> {
     await this.models.copilotContext.deleteFileEmbedding(
       this.contextId,