fix: preserve None labels in visualizations

[Iams4kura]

Summary

• Fixes: The visualizer returns no diagram when otherwise valid Mermaid output contains the word None in a node label, such as None reported.
• Root cause: VisualizerAgent.run treats any diagram output containing the substring None as the model's no-diagram sentinel, instead of accepting only the strict standalone sentinel already recognized elsewhere in the repository.
• Addresses the visualizer portion of multi_agents: strict sentinel parsers from #1909 are not used at the human gate, and the new visualizer reintroduces substring matching #2061; the separate human-approval case already has PR fix(multi_agents): exact "no" for human plan approval and bound revisions #1884.

Regression evidence

• Before: uv run --no-project --python 3.11 --with pytest python -m pytest tests/test_visualizer_none_sentinel.py -v exited 1

• After: uv run --no-project --python 3.11 --with pytest python -m pytest tests/test_visualizer_none_sentinel.py -v exited 0

Verification

• uv run --no-project --python 3.11 --with pytest --with pytest-asyncio python -m pytest tests/test_none_accept_sentinels.py tests/test_visualizer_none_sentinel.py -v
• uv run --no-project --python 3.11 python -m py_compile multi_agents/agents/visualizer.py multi_agents/agents/utils/none_sentinels.py tests/test_visualizer_none_sentinel.py tests/test_none_accept_sentinels.py
• git diff --cached --check

Scope

• 2 files changed, +65 / -1 lines

[assafelovic]

Landed in main via #2074 — commit 39215c5b (fix: land 15 issue-backed community fixes).

Your change was consolidated with related fixes into a single per-area commit so the whole set could be tested together; you're credited as co-author on it. It was applied to a clean tree and run against the full suite on its own before being stacked, and the merged result passes on Python 3.12, 3.13 and 3.14.

Thanks for the fix — closing as merged, and sorry it sat for a while.