feat(chat): stop an in-flight answer — Send becomes Stop, the partial is kept and persisted, the model stream is torn down

app/rag/agent.py

@@ -246,12 +246,19 @@ async def run_agent(
     rounds = 0
     while True:
         calls: list[ToolCallPiece] = []
-        async for piece in llm.chat_stream(
-            cast("list[dict[str, str]]", messages), tools=tools
-        ):
-            if isinstance(piece, ToolCallPiece):
-                calls.append(piece)
-            yield piece
+        # Phase 48: bind the round's stream so a consumer abandon
+        # (GeneratorExit into the yield below) tears down the in-flight
+        # model stream deterministically — not GC-dependent. Awaiting
+        # ``aclose()`` in the ``finally`` is safe because it does not
+        # yield; on a fully consumed round it is a quiet no-op.
+        stream = llm.chat_stream(cast("list[dict[str, str]]", messages), tools=tools)
+        try:
+            async for piece in stream:
+                if isinstance(piece, ToolCallPiece):
+                    calls.append(piece)
+                yield piece
+        finally:
+            await stream.aclose()
         if not calls:
             return  # the answer was streamed
         call = calls[0]  # a stream can carry several calls; run the first
@@ -287,8 +294,13 @@ async def run_agent(
                 "no-tools answer",
                 rounds,
             )
-            async for piece in llm.chat_stream(
-                cast("list[dict[str, str]]", messages), tools=None
-            ):
-                yield piece
+            # Phase 48: the forced final answer gets the same explicit
+            # teardown as the loop rounds (consumer abandon mid-final
+            # answer must still close the model's stream).
+            final = llm.chat_stream(cast("list[dict[str, str]]", messages), tools=None)
+            try:
+                async for piece in final:
+                    yield piece
+            finally:
+                await final.aclose()
             return