fix(core): accept edge_attr and other attribute dicts in visualize()

[breken-ai]

Application.visualize() and Graph.visualize() pass **engine_kwargs through to graphviz.Digraph. Dict values are merged into Burr's default attributes. Only graph_attr and node_attr have defaults, so any other attribute dict fails. For example, styling edges:

app.visualize(edge_attr={"color": "red"})
# KeyError: 'edge_attr'
#   burr/core/graph.py, in visualize: digraph_attr[g_key].update(**g_value)

Changes

• burr/core/graph.py: merge into digraph_attr.setdefault(g_key, {}), so a dict kwarg with no Burr default is passed to graphviz as given. graph_attr / node_attr still merge over Burr's defaults, as before.
• tests/core/test_graphviz_display.py: a regression that passes graph_attr, node_attr and edge_attr, and checks that each reaches the Digraph with Burr's defaults kept.

How I tested this

• On main (8161159), the new test fails with KeyError: 'edge_attr'. With the fix, it passes.
• End to end, ApplicationBuilder()...build().visualize(edge_attr={"color": "red"}) raises KeyError on main. On this branch it returns a Digraph with edge_attr == {"color": "red"}.
• pytest tests/core: 401 passed (with graphviz installed).
• black 23.11.0 (--line-length=100), isort 5.12.0 and flake8 6.1.0 at the pre-commit pins are clean on the changed files.

Notes

This is a one-line change. The merge loop has been in place since f1d935b (2024-06).

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) (no new functions)
• Placeholder code is flagged / future TODOs are captured in comments (none added)
• Project documentation has been updated if adding/changing functionality (no documented behavior changes; engine_kwargs is already documented as passed to the engine)

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AI disclosure (per the ASF Generative Tooling guidance): an AI coding tool (Claude Code, Claude Opus 5.5) wrote this change through the breken-ai account. The tool found the bug, wrote the fix and the test, and wrote this description. The commit carries a Generated-by: trailer. The diff is a small original change to existing Burr code and includes no third-party material. The red/green runs above are real, and you can re-run them from the diff. If you would rather not take AI-assisted contributions here, say so and I will close this.

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