chore(spanner): restore CrossSync parity for snapshot.py

[olavloite]

snapshot.py is generated from _async/snapshot.py, but the generated file had picked up code the async source never had. Running the generator deleted it. The drift came in via 25aba4f (PR #16488): a branch authored before the CrossSync migration but merged after it, so it edited snapshot.py as if that file were still hand-maintained.

Fixes:

• Port the _transaction_begin_event guard and the read()/execute_sql() docstrings back into _async/snapshot.py, so the generator reproduces them instead of dropping them.
• Use CrossSync.Lock / CrossSync.Event instead of raw threading, so the primitives survive the async -> sync conversion.
• De-duplicate the guard: read() and execute_sql() had identical copies, now one _wait_for_transaction_begin() helper.
• Async fix: read()/execute_sql() raised "Transaction has not begun." when a concurrent inline begin was in flight. They now wait for it, matching the sync client.
• generate.py: format with ruff instead of black. The repo uses ruff, so every run produced a formatting-only diff.
• generate.py: exit non-zero when given a file instead of a directory. It used to print "Generated 0 artifacts" and do nothing, which is how the drift went unnoticed.
• Delete snapshot_helpers.py, an orphaned generated file with a stale copy of execute_sql that nothing imports.
• Add tests for the guard, which had none.
• Speed up 4 tests that each waited out the real 30s begin timeout. They passed, just slowly; two were already slow on main. They now patch the timeout constant and assert the specific error message. Unit suite: 204s -> 57s.

Regenerating now leaves the tree unchanged:
PYTHONPATH=.cross_sync python3 .cross_sync/generate.py \ google/cloud/spanner_v1/_async/ git diff --exit-code google/cloud/spanner_v1/

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to coordinate concurrent requests beginning a transaction in the Spanner client by adding a _wait_for_transaction_begin method and an event-based synchronization mechanism to prevent race conditions. It also updates the cross-sync generation script to use ruff for formatting generated artifacts and includes various documentation and cleanup improvements across the package. I have no feedback to provide on this pull request.