[fix] support AGUI reasoning role variants

[pandego]

Summary

• make AGUI reasoning start events choose a role supported by the installed ag-ui-protocol schema
• keep reasoning streams compatible across protocol versions that expect either assistant or reasoning
• add regression assertions for reasoning start events in the AGUI unit tests

Fixes #7556

Type of change

• Bug fix
• New feature
• Breaking change
• Improvement
• Model update
• Other:

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Checklist

• Code complies with style guidelines
• Ran format/validation scripts (./scripts/format.sh and ./scripts/validate.sh)
• Self-review completed
• Documentation updated (comments, docstrings)
• Examples and guides: Relevant cookbook examples have been included or updated (if applicable)
• Tested in clean environment
• Tests added/updated (if applicable)

Duplicate and AI-Generated PR Check

• I have searched existing open pull requests and confirmed that no other PR already addresses this issue
• If a similar PR exists, I have explained below why this PR is a better approach
• Check if this PR was entirely AI-generated (by Copilot, Claude Code, Cursor, etc.)

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Additional Notes

Validation run in this workspace:

• ruff format libs/agno/agno/os/interfaces/agui/utils.py libs/agno/tests/unit/app/test_agui_app.py
• ruff check libs/agno/agno/os/interfaces/agui/utils.py libs/agno/tests/unit/app/test_agui_app.py
• PYTHONPATH=libs/agno /Users/kitt/.openclaw/workspace/agents/charlie/repos/agno/libs/agno/.venv/bin/python -m pytest libs/agno/tests/unit/app/test_agui_app.py -k 'reasoning' -q

I did not run ./scripts/validate.sh because fresh uv resolution in this repo currently fails on unrelated optional dependency conflicts (a2a-sdk vs brave-search) in this environment.