* Document Python 3.10 client RC reproducibility and add verify helper
Documents that the Python client reproducible build must run under Python 3.10
(the trigger_dag_run_post_body.py AST re-emit is interpreter-version sensitive),
and adds dev/verify_python_client_rc.sh — a headless contributor smoke test that
boots airflow standalone inside breeze and runs test_python_client.py against
the live API.
* Fail fast when building Python client under non-default Python
prepare-python-client must run under DEFAULT_PYTHON_MAJOR_MINOR_VERSION
(3.10): the client generator re-emits trigger_dag_run_post_body.py with
ast.unparse, which uses the running interpreter's grammar, so any other
Python yields a non-reproducible client. The existing guard only ran
after full code generation (which itself can fail or emit nothing under
the wrong Python), so the command never failed cleanly.
Hoist the check to the start of the command so it exits immediately with
actionable guidance, and document that the command now refuses to run
under any other Python.
