fix: close unfetched async commands

[nanookclaw]

Summary

Cursor.close() now frees an active async command id when no result set has been fetched yet. This covers the execute_async() followed by close() path from #791, where active_command_id is set but active_result_set is still None.

When a result set exists, close still delegates to active_result_set.close() so that path can perform its existing backend cleanup without a duplicate close_command call. The cursor clears active_command_id in a finally block either way.

Tests

• PYTHONPATH=src uv run --with pytest --with thrift --with pandas --with lz4 --with requests --with oauthlib --with openpyxl --with urllib3 --with python-dateutil --with pyjwt --with pybreaker --python 3.12 --env-file test.env.example python -m pytest tests/unit/test_client.py -k 'close_closes_active_command_without_result_set or close_delegates_to_active_result_set_without_double_closing_command or closing_result_set_hard_closes_commands or closing_connection_closes_commands'
• uv run --with black==22.3.0 --python 3.12 black --check src/databricks/sql/client.py tests/unit/test_client.py
• git diff --check

Closes #791