fix fx/static resampling path for mrsofc

[rbeucher]

what this does

this fixes the fx/static path so we do not try to do temporal resampling logic on time-invariant targets.

changes

• add explicit static-target handling in validate_and_resample_if_needed()
• accept datasets with no time coordinate or a singleton time
• raise a clear IncompatibleFrequencyError if an fx target gets truly time-varying input
• add unit tests in tests/unit/test_temporal_resampling.py for these cases
• keep existing resampling behavior unchanged for non-fx targets

why

we hit a failure in the cmip7 land integration case (mrsofc) where the pipeline reached resampling validation for an fx variable and blew up on time-frequency handling.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.7%. Comparing base (24d23cc) to head (174482d).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #494   +/-   ##
=====================================
  Coverage   78.7%   78.7%           
=====================================
  Files         33      33           
  Lines       6548    6557    +9     
  Branches    1239    1242    +3     
=====================================
+ Hits        5153    5162    +9     
  Misses      1127    1127           
  Partials     268     268           

Flag	Coverage Δ
unit	78.7% <100.0%> (+<0.1%)	⬆️

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