fix(visualization): keep fillcolor on handoff nodes by merging style attributes

[comp]

Summary

Handoff and sub-agent nodes in the agent visualization are meant to render as filled lightyellow boxes (matching the root agent node), but they currently render as empty/unfilled boxes.

The node attribute list sets style twice: style=filled, style=rounded. In the DOT language a repeated attribute takes the last value, so style resolves to rounded only and filled is dropped. Because fillcolor only takes effect when filled is present in style, the fillcolor=lightyellow was silently ignored.

This merges the two into a single valid value, style="filled,rounded", so the nodes are both filled and rounded as intended. Applies to both the Handoff and Agent handoff branches in get_all_nodes.

Test plan

Verified against the actual Graphviz renderer (dot) by rendering a node both ways to SVG and inspecting the node polygon fill:

• Before (style=filled, style=rounded) -> fill="none" (unfilled)
• After (style="filled,rounded") -> fill="lightyellow" (filled, as intended)

Updated the existing tests/test_visualization.py expectations (which asserted the old DOT string) to match the corrected output. Ran the standard checks:

• uv run ruff format --check — pass
• uv run ruff check — pass
• uv run mypy src/agents/extensions/visualization.py — pass
• uv run pytest tests/test_visualization.py — 10 passed

Issue number

N/A

Checks

• I've added new tests, if relevant (updated existing visualization assertions to cover the corrected output)
• I've run .agents/skills/code-change-verification/scripts/run.sh (ran the equivalent stack manually: ruff format/check, mypy, pytest — see test plan)
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR — N/A

[seratch]

Thanks. This is indeed a nice improvement 👍