Introduce `wr.athena.start_query_executions` as a parallelized variant of
`start_query_execution`. It allows submitting multiple queries in one call,
with support for:
- Sequential or threaded submission (`use_threads`)
- Lazy or eager consumption of results (`as_iterator`)
- Per-query `client_request_token` (string or list)
- Optional workgroup checks (`check_workgroup`, `enforce_workgroup`)
- Full Athena cache integration
This improves performance when dispatching batches of queries by reducing
workgroup lookups and enabling concurrent execution.

...nd parallel wait
- Simplified client_request_token handling:
 - Removed manual padding/truncation.
 - Let Athena enforce length constraints.
 - Tokens generated as `<base_token>-<index>` or provided as list.
- Improved wait logic:
 - Added optional wait handling directly inside _submit.
 - Queries can now be waited in parallel with submission (reduced overhead).
- Configurable default threads:
 - Replaced hardcoded defaults with os.cpu_count().
 - Added support for AWSWRANGLER_THREADS_DEFAULT env var override.

- Removed unused `reduce` import from Athena module.
- Applied ruff formatting to `start_query_executions`.
- Fixed static check issues to pass CI.
- Added ruff check on Athena tests file.