chore: updating CONTRIBUTING.md with missing extra

[jackwotherspoon]

Contributing doc says to do the following:

uv sync --extra test --extra eval
pytest ./tests/unittests

If you follow this the tests will fail:

tests/unittests/a2a/executor/test_task_result_aggregator.py:27: in <module>
    from a2a.types import Message
E   ModuleNotFoundError: No module named 'a2a'

This makes sense since the a2a package is not part of ADK's core dep, it is an extra:

adk-python/pyproject.toml

Lines 79 to 83 in e79651c

	a2a = [
	# go/keep-sorted start
	"a2a-sdk>=0.2.7;python_version>='3.10'"
	# go/keep-sorted end
	]

Thus for a2a tests to pass we must include the extra in the sync command:

uv sync --extra test --extra eval --extra a2a
pytest ./tests/unittests

[jackwotherspoon]

@seanzhou1023 mind giving this a stamp? 😄