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/templates/default-pipeline-config.json

@@ -17,6 +17,10 @@
             "prefix": [],
             "regexp": []
         },
+        "message-aggregation": {
+            "enabled": false,
+            "delay": 1.5
+        },
         "misc": {
             "combine-quote-message": true
         }