GeezerVison

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Fixes #18375 🦕

Database.run_in_transaction sets _local.transaction_running = True and then calls get_session() outside the try/finally that resets the flag. If checkout raises, for example ServiceUnavailable while Spanner is down, the flag is never cleared. Every later call on that thread then fails with RuntimeError: Spanner does not support nested transactions, even after Spanner recovers.

The change moves session checkout inside the try, and only calls put_session when a session was actually obtained, so None is never returned to the sessions manager. The async implementation in _async/database.py had the same shape. I fixed it there and made the identical change to the generated sync module, and checked that the CrossSync conversion of the async method matches the sync one.

Tests, sync and async:

• test_run_in_transaction_session_error: the first checkout raises ServiceUnavailable, and a second call succeeds. The failed attempt doesn't call put_session, the flag is reset, and the session from the second call is returned.
• test_run_in_transaction_callback_error: when the callback raises, the flag is reset and the session is still returned. This pins the existing behaviour.

Without the fix, both session_error tests fail with the nested-transaction RuntimeError. With it, tests/unit passes, and ruff and flake8 are clean.

@mukeshr06 reproduced this in September and mentioned preparing a fix. I haven't seen a PR since, so here is one. Happy to close this in favour of theirs if it's close.