fix(langgraph): offload sync remote tool coroutines

[fashkl]

Problem

This mirrors the upstream Oracle contribution in oracle#229 and tracks oracle#228.

LangGraph RemoteTool conversion exposes a coroutine for sync remote-tool callables, but the coroutine currently calls the synchronous function directly. For RemoteTool, that function executes httpx.request(...), so async LangGraph execution can block the event loop while waiting on network I/O.

This is worse than leaving StructuredTool.coroutine unset. LangChain's own StructuredTool._arun checks self.coroutine first; when it is None, it falls back to the base async implementation, whose source comment says it is expected to delegate _run to a separate thread. By supplying a fake async wrapper, pyagentspec bypasses that safe default and runs blocking sync I/O on the event-loop thread.

This also makes RemoteTool inconsistent with the existing ApiNode executor split in this codebase: ApiNodeExecutor._execute() uses sync httpx.request(...), while ApiNodeExecutor._aexecute() uses httpx.AsyncClient. RemoteTool is the outlier because its async entrypoint is only async-shaped, not async-safe.

In services with event-loop based liveness/readiness handling, a slow or stuck remote tool request can therefore freeze unrelated async work long enough to trip health checks and restart the service.

Fix

Offload synchronous structured-tool coroutine execution with asyncio.to_thread(...) while preserving native async callables unchanged.

This is a small general safety net for sync callables that pyagentspec intentionally exposes through a coroutine, including the current RemoteTool and ClientTool conversion paths. A future native async RemoteTool implementation using httpx.AsyncClient would still be compatible with this change and could further reduce thread usage for RemoteTool specifically.

Testing

• Added test_remote_tool_coroutine_does_not_block_event_loop
• Ran SKIP_LLM_TESTS=1 uv run python -m pytest -q tests/adapters/langgraph/test_tools.py -k 'remote_tool_coroutine_does_not_block_event_loop or async_server_tool_callable_converts_to_structured_tool_coroutine'
• Ran SKIP_LLM_TESTS=1 uv run python -m pytest -q tests/adapters/langgraph/test_tools.py
• Ran black and isort checks for the changed files

Commit is signed off per the Oracle contribution guide.