fix(agent): close length recovery lifecycle gaps

nanobot/agent/turn_delivery.py

@@ -126,6 +126,7 @@ class TurnDelivery:
     lifecycle_message: InboundMessage = field(init=False)
     _stream_base_id: str | None = field(init=False, default=None)
     _stream_segment: int = field(init=False, default=0)
+    _stream_open: bool = field(init=False, default=False)
 
     def __post_init__(self) -> None:
         self.delivery_message = dataclasses.replace(
@@ -284,6 +285,7 @@ class TurnDelivery:
                 metadata=self.delivery_message.metadata,
             )
         )
+        self._stream_open = True
 
     async def _publish_stream_end(
         self,
@@ -303,5 +305,11 @@ class TurnDelivery:
                 metadata=self.delivery_message.metadata,
             )
         )
+        self._stream_open = merge_next
         if not merge_next:
             self._stream_segment += 1
+
+    async def abort_stream(self) -> None:
+        """Close an interrupted stream so stateful channels can release its buffer."""
+        if self._stream_open:
+            await self._publish_stream_end()