Reuse Athena Spark sessions across Spark tests

[laughingman7743]

Use case

Spark tests cost about $20 of the September bill (58 DPU-hours; see #834).

• The spark_cursor and async_spark_cursor fixtures in tests/pyathena/conftest.py and aio_spark_cursor in tests/pyathena/aio/conftest.py are function-scoped.
• Every Spark test starts a new Athena Spark session and waits for it to become idle.
• The cursor's close() terminates the session.

So the DPU cost of starting a session is paid once per test.

Since #837, Spark tests run on pull requests only when Spark code or tests change, plus weekly and on dispatch. The Spark lifecycle work tracked in #791 changes those files, so they run often while that work is in progress.

Proposed change

Reuse Spark sessions across tests where isolation allows it:

• Provide a module- or worker-scoped Spark connection/session fixture for tests that only run calculations.
• Keep function-scoped sessions for tests that exercise session lifecycle: creation failure, termination, cancellation, and close() behavior.
• Check how this interacts with the Improve Spark cursor session ownership and lifecycle handling #791 changes (for example, terminate_session_on_close) so the tests still cover the lifecycle contracts.

Validation plan (if implementing)

• Run the Spark tests (tests/pyathena/spark/, tests/pyathena/aio/spark/) against the pyathena-spark work group, and confirm the same results as before.
• Compare the number of sessions started (ListSessions for the work group during the run) and the DPU-hours before and after.