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/models/copilot-session.ts

@@ -396,7 +396,10 @@ export class CopilotSessionModel extends BaseModel {
   }
 
   @Transactional()
-  async update(options: UpdateChatSessionOptions): Promise<string> {
+  async update(
+    options: UpdateChatSessionOptions,
+    internalCall = false
+  ): Promise<string> {
     const { userId, sessionId, docId, promptName, pinned, title } = options;
     const session = await this.getExists(
       sessionId,
@@ -415,14 +418,16 @@ export class CopilotSessionModel extends BaseModel {
     }
 
     // not allow to update action session
-    if (session.prompt.action) {
-      throw new CopilotSessionInvalidInput(
-        `Cannot update action: ${session.id}`
-      );
-    } else if (docId && session.parentSessionId) {
-      throw new CopilotSessionInvalidInput(
-        `Cannot update docId for forked session: ${session.id}`
-      );
+    if (!internalCall) {
+      if (session.prompt.action) {
+        throw new CopilotSessionInvalidInput(
+          `Cannot update action: ${session.id}`
+        );
+      } else if (docId && session.parentSessionId) {
+        throw new CopilotSessionInvalidInput(
+          `Cannot update docId for forked session: ${session.id}`
+        );
+      }
     }
 
     if (promptName) {

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
+      );
     }
   }