Remove async stored procedure client parameter gate

[sfc-gh-gshe]

Summary

Remove Snowpark Python's client-side dependency on ENABLE_ASYNC_QUERY_IN_PYTHON_STORED_PROCS. Async queries in Python stored procedures are already enabled by default and the server-side parameter is being removed in snowflake-eng/snowflake#518256.

This change:

• removes guards from non-blocking execution and Session.create_async_job
• always uses the stored-procedure cancellation path from AsyncJob.cancel
• allows temporary-table cleanup to proceed in stored procedures
• removes obsolete mock parameter values and stored-procedure integration-test skips
• adds focused regression coverage and a changelog entry

Rollout ordering

This client change should land before snowflake-eng/snowflake#518256. While the server parameter still exists, it is server-controlled, defaults to true, and has no observed deployment overrides, so removing the client checks preserves current behavior. Once a release containing this change is available to stored-procedure runtimes, the server parameter can be removed safely.

Testing

• PYTHONPATH=src:tests pytest tests/unit/test_async_job.py tests/unit/test_server_connection.py tests/unit/test_session.py --confcutdir=tests/unit -q — 71 passed
• pre-commit hooks on all changed files — passed