drop_incomplete support in SeasonGrouper

[dhruvak001]

• Closes Support drop_incomplete kwarg with SeasonGrouper #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.

---

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.

---

[welcome]

Thank you for opening this pull request! It may take us a few days to respond here, so thank you for being patient.
If you have questions, some answers may be found in our contributing guidelines.

[dcherian]

Is there code in SeasonResampler we could reuse here?

Can you add a couple of tests please?