Fix collect visualization edge styling

[Probablism]

Fixes #556

This narrows collect edge styling in graph visualization so only the dependency annotated as Collect[...] is drawn with the collect crow-foot. Ordinary scalar inputs into the same collect node remain normal edges.

Changes

• Apply collect edge styling only when the edge feeds the target node's actual Collect[...] dependency.
• Keep collect node styling and expand edge styling unchanged.
• Add a regression test for the multi-argument collect visualization case.

How I tested this

• MPLCONFIGDIR=.mplconfig XDG_CACHE_HOME=.cache UV_CACHE_DIR=.uv-cache ./.uv-tools/bin/uv run pytest tests/test_graph.py -q
• UV_CACHE_DIR=.uv-cache ./.uv-tools/bin/uv run ruff check hamilton/graph.py tests/test_graph.py
• UV_CACHE_DIR=.uv-cache ./.uv-tools/bin/uv run ruff format --check hamilton/graph.py tests/test_graph.py
• git diff --check

Notes

This is limited to visualization edge styling. It does not change dynamic execution or grouping semantics.

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 focused checks listed above & 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.