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.

Testing

Passed:

uv run python -m 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 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

Attempted tox:

env PATH=/opt/homebrew/bin:/Users/jordanchen/.local/bin:/usr/bin:/bin:/usr/sbin:/sbin uv run tox -r -e py312,py313

Result:

py312: 11 failed, 7473 passed, 19 skipped, 31 xfailed, 9 xpassed
py313: 11 failed, 7473 passed, 19 skipped, 31 xfailed, 9 xpassed

The tox failures are all in tests/unittests/models/test_interactions_utils.py
and fail because the installed google.genai.interactions.FunctionCallStep
does not expose the signature attribute expected by
src/google/adk/models/interactions_utils.py. The A2A and CLI tests touched by
this PR pass in the same environment.