fix: allow gamePks schedule queries without date

[Mattsface]

Why

Fixes #245.

Schedule requests using gamePks were incorrectly requiring a date. Both get_schedule() and get_scheduled_games_by_date() would return None before the request ever reached the MLB API when gamePks was provided without a date.

The MLB schedule endpoint supports querying by gamePks without requiring a date.

What

• Allow gamePks schedule queries without a date in get_schedule()
• Allow gamePks schedule queries without a date in get_scheduled_games_by_date()
• Preserve the existing behavior when neither a date nor gamePks is provided
• Add regression tests covering gamePks-only requests for both methods

Tests

Added regression tests in tests/test_schedule_regressions.py.

The tests verify that:

• A gamePks-only request reaches the data adapter
• gamePks is passed through correctly
• The default sportId is preserved
• No date parameters are added to the request

The tests were confirmed to fail before the fix because the adapter was never called, and pass after the change.

Full test suite passes.

Risk and impact

Risk: Minimal

The change is limited to the schedule parameter validation logic and only changes behavior when gamePks is supplied without a date. Existing date and date-range behavior is unchanged.

If something does go wrong, the impact should be limited to schedule queries involving gamePks. The added regression tests cover the affected code paths.