Add LangGraph workflow streams sample

[brianstrauch]

What

Adds a Graph-API sample demonstrating the LangGraph plugin's recently-added Workflow Streams support, under langgraph_plugin/graph_api/streaming/.

It combines both streaming patterns in one workflow:

• Node token streaming — the write_story node emits live tokens via LangGraph's get_stream_writer(), routed onto the tokens topic by the plugin's streaming_topic.
• Workflow-side astream publish — the workflow drives the graph with app.astream(...) and publishes each node-completion chunk onto a progress topic it owns via WorkflowStream.topic().publish().

A single client subscribes to all topics with WorkflowStreamClient and demultiplexes on item.topic. The workflow waits on an ack_stream signal before returning so the stream survives until the client finishes consuming it.

Dependency changes

• Bumps the langgraph group to temporalio>=1.28.0 (where workflow streams ship).
• Drops the now-obsolete langsmith<0.7.34 uv constraint — 1.28.0 requires langsmith>=0.7.34, and the pin was specific to the 1.27.2 langsmith patch. Existing langgraph tests still pass on the resulting langsmith 0.8.9.

Also adds langgraph_plugin/ code owners.

Testing

• Added tests/langgraph_plugin/streaming_test.py; the full tests/langgraph_plugin/ suite passes (12 passed) on temporalio 1.28.0 / langsmith 0.8.9.
• ruff format/import checks and mypy are clean for the new files.

🤖 Generated with Claude Code