Move crewai.a2a into lib/crewai-a2a as its own workspace package, importable as crewai_a2a. The crewai[a2a] extra now pulls in crewai-a2a, which owns a2a-sdk, httpx-auth, httpx-sse, and aiocache.
crewai.a2a stays importable. Its __init__ is a compat shim that installs a meta-path finder routing crewai.a2a.* to crewai_a2a.*, so existing user code keeps working untouched.
a2a tests and cassettes moved alongside the package under lib/crewai-a2a/tests/. Added that path to the mypy and ruff per-file-ignores lists to match the other test dirs.
