feat(a2a): Infer agent cards from ADK agents

[jordanchendev]

Summary

Fixes #2237.

This lets get_fast_api_app(..., a2a=True) expose standard ADK agent
directories even when they do not include an explicit agent.json. Existing
agent.json files still take precedence; otherwise the loaded ADK agent is
used to build an AgentCard.

The loaded agent is normalized to the BaseAgent | Workflow shape accepted by
AgentCardBuilder, so App.root_agent is validated before card generation.

Testing

Checked:

uv run mypy . | grep "error:" | sed 's/:\([0-9]\+\):/::/g' | sort > /private/tmp/adk_pr_errors_after.txt || true

Result:

2587 mypy error lines collected from the PR branch.
No fast_api.py AgentCardBuilder arg-type error was present.
No tests/unittests/cli/test_fast_api_a2a.py error was present.

Passed:

uv run pytest tests/unittests/cli/test_fast_api_a2a.py tests/unittests/cli/test_fast_api.py -k a2a tests/unittests/a2a/utils/test_agent_card_builder.py tests/unittests/a2a/utils/test_agent_to_a2a.py -q

Result:

113 passed, 75 deselected, 61 warnings

Passed:

uv run pytest tests/unittests/models/test_interactions_utils.py -q

Result:

81 passed, 4 warnings

Passed:

uv run pre-commit run --all-files

Result:

check yaml...............................................................Passed
fix end of files.........................................................Passed
trim trailing whitespace.................................................Passed
pyproject-fmt............................................................Passed
ruff (legacy alias)......................................................Passed
isort....................................................................Passed
pyink....................................................................Passed
addlicense...............................................................Passed
Check new Python files have _ prefix.....................................Passed
mdformat.................................................................Passed

Not rerun after rebasing:

tox

I previously ran py312,py313 tox and hit failures in
tests/unittests/models/test_interactions_utils.py on the old base. After
rebasing onto latest origin/main, that previously failing test file now
passes as shown above.

[rohityan]

Hi @jordanchendev , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @xuanyang15 , can you please review this.