fix(ctxmgr): EnsureChatSession 归属 COALESCE 改为已有值优先（R-1c）

防止跨用户通过 upsert/消息落库覆盖已绑定的 chat_sessions.user_id；新增 EnsureChatSessionOwned。

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/ctxmgr/chat_history.go

@@ -32,7 +32,7 @@ func (m *ContextManager) EnsureChatSession(ctx context.Context, sessionID string
 			VALUES ($1, $2, $3, $4, now(), now())
 			ON CONFLICT (session_id) DO UPDATE SET
 				title = COALESCE(NULLIF($4, ''), chat_sessions.title),
-				user_id = COALESCE($2, chat_sessions.user_id),
+				user_id = COALESCE(chat_sessions.user_id, $2),
 				project_id = COALESCE($3, chat_sessions.project_id),
 				updated_at = now()
 		`, sessionID, uid, pid, title)
@@ -79,7 +79,7 @@ func (m *ContextManager) EnsureChatSession(ctx context.Context, sessionID string
 		if title != "" {
 			sess.Title = title
 		}
-		if userID > 0 {
+		if userID > 0 && sess.UserID <= 0 {
 			sess.UserID = userID
 		}
 		if projectID > 0 {
@@ -97,6 +97,22 @@ func (m *ContextManager) EnsureChatSession(ctx context.Context, sessionID string
 	return &copy, nil
 }
 
+// ErrSessionOwnerMismatch 表示调用者试图操作已绑定给其他用户的会话。
+var ErrSessionOwnerMismatch = errors.New("ctxmgr: session owner mismatch")
+
+// EnsureChatSessionOwned 在写入前校验归属：已绑定且不匹配调用者时返回
+// ErrSessionOwnerMismatch；未绑定或本就是调用者本人时正常落地。
+func (m *ContextManager) EnsureChatSessionOwned(
+	ctx context.Context, sessionID string, callerUserID, projectID int64, title string,
+) (*ChatSessionSummary, error) {
+	if callerUserID > 0 {
+		if owner, err := m.GetSessionOwnerID(ctx, sessionID); err == nil && owner > 0 && owner != callerUserID {
+			return nil, ErrSessionOwnerMismatch
+		}
+	}
+	return m.EnsureChatSession(ctx, sessionID, callerUserID, projectID, title)
+}
+
 // ListChatSessions 分页查询历史会话列表
 func (m *ContextManager) ListChatSessions(ctx context.Context, filter SessionFilter) ([]ChatSessionSummary, error) {
 	if filter.Limit <= 0 {

internal/ctxmgr/manager.go

@@ -190,13 +190,13 @@ func (m *ContextManager) RecordUserMessageWithID(ctx context.Context, sessionID
 			}
 		}
 
-		// 确保会话存在
+		// 确保会话存在（user_id 已有值优先，防止跨用户覆盖归属）
 		_, _ = m.pool.Exec(ctx, `
 			INSERT INTO chat_sessions (session_id, user_id, project_id, title, created_at, updated_at)
 			VALUES ($1, $2, $3, '对话协同会话', now(), now())
 			ON CONFLICT (session_id) DO UPDATE SET 
 				updated_at = now(),
-				user_id = COALESCE($2, chat_sessions.user_id),
+				user_id = COALESCE(chat_sessions.user_id, $2),
 				project_id = COALESCE($3, chat_sessions.project_id)
 		`, sessionID, uid, pid)