Style async nodes in graph visualizations

[officialasishkumar]

Adds a distinct graphviz style and legend entry for async-backed Hamilton nodes so DAG visualizations can signal when AsyncDriver is needed for execution.

Changes

• Added async callable detection in graphviz visualization generation.
• Applied a distinct async node style and included it in the legend ordering.
• Added regression coverage asserting async nodes and their legend entry render with the async style.

How I tested this

• uv run --with pytest --with graphviz pytest tests/test_graph.py::test_create_graphviz_graph_styles_async_nodes
• uv run ruff format --check hamilton/graph.py tests/test_graph.py
• uv run ruff check hamilton/graph.py tests/test_graph.py

Notes

Fixes #1182.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[skrawcz]

Can you provide a screenshot please?