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
         }

src/langbot/templates/metadata/pipeline/trigger.yaml

@@ -123,6 +123,34 @@ stages:
         type: array[string]
         required: true
         default: []
+  - name: message-aggregation
+    label:
+      en_US: Message Aggregation
+      zh_Hans: 消息聚合
+    description:
+      en_US: When a user sends multiple messages consecutively, wait for a period and merge them into one before processing
+      zh_Hans: 当用户连续发送多条消息时，等待一段时间后合并为一条消息再处理（防抖）
+    config:
+      - name: enabled
+        label:
+          en_US: Enable Message Aggregation
+          zh_Hans: 启用消息聚合
+        description:
+          en_US: If enabled, consecutive messages from the same user will be merged after a delay
+          zh_Hans: 如果启用，同一用户连续发送的消息将在延迟后合并处理
+        type: boolean
+        required: true
+        default: false
+      - name: delay
+        label:
+          en_US: Aggregation Delay (seconds)
+          zh_Hans: 聚合延迟（秒）
+        description:
+          en_US: 'Wait time before merging messages. Range: 1.0-10.0 seconds.'
+          zh_Hans: '合并消息前的等待时间。范围：1.0-10.0 秒。'
+        type: float
+        required: true
+        default: 1.5
   - name: misc
     label:
       en_US: Misc