feat: message aggregator (#1985)

* feat: aggregator

* fix: resolve deadlock, mutation, and safety issues in message aggregator

- Fix deadlock: don't await cancelled timer tasks inside the lock;
  _flush_buffer acquires the same lock, causing a deadlock cycle
- Fix message_event mutation: keep original message_event unmodified
  to preserve message_id/metadata for reply/quote; only pass merged
  message_chain separately
- Fix Plain positional arg: Plain('\n') → Plain(text='\n')
- Fix float() ValueError: wrap delay cast in try/except
- Add MAX_BUFFER_MESSAGES (10) cap to prevent unbounded buffer growth
- Default enabled to false to avoid surprising latency on upgrade
- Fix flush_all: cancel all timers under one lock acquisition, then
  flush outside the lock to avoid deadlock

---------

Co-authored-by: RockChinQ <rockchinq@gmail.com>

src/langbot/pkg/core/app.py

@@ -15,6 +15,7 @@ from ..command import cmdmgr
 from ..plugin import connector as plugin_connector
 from ..pipeline import pool
 from ..pipeline import controller, pipelinemgr
+from ..pipeline import aggregator as message_aggregator
 from ..utils import version as version_mgr, proxy as proxy_mgr
 from ..persistence import mgr as persistencemgr
 from ..api.http.controller import main as http_controller
@@ -96,6 +97,8 @@ class Application:
 
     query_pool: pool.QueryPool = None
 
+    msg_aggregator: message_aggregator.MessageAggregator = None
+
     ctrl: controller.Controller = None
 
     pipeline_mgr: pipelinemgr.PipelineManager = None