drop_incomplete support in SeasonGrouper

[dhruvak001]

• [x] Closes #10426
• [ ] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

SeasonGrouper Enhancements

Enhanced the SeasonGrouper class to support conditional omission of incomplete seasons—now aligned with behavior in SeasonResampler.

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Summary of Changes

1. drop_incomplete Keyword Argument

• Introduced a new boolean parameter drop_incomplete, defaulting to False for backward compatibility.
• Enables users to choose whether incomplete seasons should be excluded during grouping.

2. Revised factorize Method

• Core logic for dropping incomplete seasons has been moved into the factorize method.
• When drop_incomplete=True, it:
  • Iterates over each year-series chunk.
  • Verifies that all expected months in the season are present.
  • Excludes any data for that season-year pair if months are missing.

3. Updated reset Method

• Ensures that the drop_incomplete setting correctly propagates when resetting or re-initializing the grouper.

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[dhruvak001]

@dcherian the failing tests seems to be unrelated to changes in the PR, right?

[dcherian]

Yes.

Apologies, I am quite busy at the moment and can only review this in two weeks.

@tomvothecoder if you happen to have tests for grouping with drop_incomplete, those would be very helpful here.