fix(langgraph): reasoning Start needs non-null style + native async tool

- _langgraph_sync.py: reasoning StreamTaskMessageStart now sets style="active".
  The AgentEx server's StreamTaskMessageStartEntity rejects reasoning.style=None
  (enum), which killed the sync stream — breaking harness_langgraph and the
  pre-existing 030_langgraph that share this emitter.
- temporal harness tools.py: give get_weather a native async coroutine so
  tools_node's `await tool.ainvoke(...)` runs on the workflow loop instead of
  LangChain's run_in_executor fallback (NotImplementedError in the deterministic
  Temporal workflow sandbox).
- test_langgraph_sync.py: assert the reasoning Start carries a non-null style.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: declan-scale

examples/tutorials/10_async/10_temporal/harness_langgraph/project/tools.py

@@ -15,9 +15,22 @@ def get_weather(city: str) -> str:
     return f"The weather in {city} is sunny and 72°F"
 
 
+async def aget_weather(city: str) -> str:
+    """Native async tool entrypoint.
+
+    ``tools_node`` runs inline in the Temporal workflow and invokes tools via
+    ``tool.ainvoke``. A sync-only tool forces LangChain to bridge through
+    ``run_in_executor`` (a thread pool), which the deterministic Temporal
+    workflow event loop forbids (``NotImplementedError``). Providing a real
+    coroutine keeps tool execution on the workflow loop.
+    """
+    return get_weather(city)
+
+
 weather_tool = Tool(
     name="get_weather",
     func=get_weather,
+    coroutine=aget_weather,
     description="Get the current weather for a city. Input should be a city name.",
 )
 

src/agentex/lib/adk/_modules/_langgraph_sync.py

@@ -149,7 +149,9 @@ async def convert_langgraph_to_agentex_events(
                             yield StreamTaskMessageStart(
                                 type="start",
                                 index=message_index,
-                                content=ReasoningContent(type="reasoning", author="agent", summary=[], content=[]),
+                                content=ReasoningContent(
+                                    type="reasoning", author="agent", summary=[], content=[], style="active"
+                                ),
                             )
                             reasoning_streaming = True
                             reasoning_content_index = 0

tests/lib/adk/test_langgraph_sync.py

@@ -101,6 +101,10 @@ async def test_reasoning_block_start_wraps_reasoning_content(self):
         starts = [e for e in out if isinstance(e, StreamTaskMessageStart)]
         assert len(starts) == 1
         assert isinstance(starts[0].content, ReasoningContent), "reasoning Start must wrap ReasoningContent"
+        # `style` must be a non-null MessageStyle: the AgentEx server's
+        # StreamTaskMessageStartEntity rejects `reasoning.style=None` (enum), which
+        # would kill the stream. Match the conformance fixture's canonical value.
+        assert starts[0].content.style == "active", "reasoning Start must set a non-null style ('active')"
         # Pull content_delta inside the comprehension so the isinstance narrows the
         # delta union (narrowing would not survive a later attribute access).
         reasoning_delta_texts = [