Support rechunking to seasonal frequency with SeasonalResampler

[dhruvak001]

• Closes Support rechunking to seasonal frequency with SeasonalResampler #10425
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

users could not use SeasonResampler for chunking operations in xarray, despite it being a natural fit for seasonal data analysis. When attempting ds.chunk(time=SeasonResampler(["DJF", "MAMJ", "JAS", "ON"])), users encountered obscure errors because the chunking logic was hardcoded to only work with TimeResampler objects. This limitation prevented efficient seasonal analysis workflows and forced users to use workarounds or manual chunking strategies.

Now Added a generalized chunking approach by adding a resolve_chunks method to the Resampler base class and updating the chunking logic to work with all Resampler objects, not just TimeResampler. We also added a _for_chunking method to SeasonResampler that ensures drop_incomplete=False during chunking operations to prevent silent data loss. The solution maintains full backward compatibility with existing TimeResampler functionality while enabling seamless seasonal chunking

[dcherian]

This doesn't raise any more, now that I moved it here. That's because the Resampler isn't raising an error, but our check in dataset.chunk was. Let's raise a nice error message here. The check could be len("".join(self.seasons)) == 12

[dcherian]

Can you please write out the chunks tuple here (and below)?

[dcherian]

Almost there! I have a couple of small requests.

@keewis does this API look like it can work with the DGGSResampler idea we talked about? You should be able to rechunk to a "zoom level" so that a chunk completely contains an integer number of "parent cells".