fix(persistence): separate retryable rejection errors (#6423)

app/application/messaging/chat.py

@@ -12,10 +12,7 @@ from app.application.database import (
     AsyncDatabaseExecutor,
 )
 from app.schemas.agent import AgentChatSessionDetail, AgentChatSessionSummary
-from app.schemas.exception import (
-    DatabaseWorkerClosedError,
-    DatabaseWorkerOverloadedError,
-)
+from app.schemas.exception import AgentChatPersistenceUnavailableError
 from app.runtime.observability import record_metric
 
 
@@ -364,14 +361,16 @@ class AgentChatPersistenceService:
         """在线程 worker 内完成同步写入并丢弃仓储对象返回值。"""
         # 同时限制全局和单会话等待量，避免一个热点会话占满总 admission 后饿死其他会话。
         if self._closing:
-            raise DatabaseWorkerClosedError("AgentChat 持久化服务当前不可接收任务")
+            raise AgentChatPersistenceUnavailableError(
+                "AgentChat 持久化服务当前不可接收任务"
+            )
         session_pending = self._pending_by_session.get(session_id, 0)
         if (
             self._pending_writes >= self._capacity
             or session_pending >= self._session_capacity
         ):
             record_metric("agent.chat.persistence.rejected")
-            raise DatabaseWorkerOverloadedError(
+            raise AgentChatPersistenceUnavailableError(
                 f"AgentChat 写入容量已用尽（全局上限 {self._capacity}，"
                 f"单会话上限 {self._session_capacity}）"
             )