feat(server): allow cleanup session for deleted docs (#13720)

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

## Summary by CodeRabbit

* **Bug Fixes**
* Resolved occasional errors when removing document links from sessions,
ensuring cleanup completes reliably.
* Improved reliability during maintenance actions by preventing
unnecessary validation failures in system-initiated updates, while
preserving existing checks for user-initiated changes.

* **Chores**
* Internal adjustments to the session update flow to better support
maintenance operations without affecting user-facing behavior.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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

@@ -636,11 +636,10 @@ export class ChatSessionService {
       })
       .then(s => s.map(s => [s.userId, s.id]));
     for (const [userId, sessionId] of sessionIds) {
-      await this.models.copilotSession.update({
-        userId,
-        sessionId,
-        docId: null,
-      });
+      await this.models.copilotSession.update(
+        { userId, sessionId, docId: null },
+        true
+      );
     }
   }