Load bundled areacello in ACCESS-ESM1-6 ocean and sea-ice mappings

[rbeucher]

This updates the ACCESS-ESM1-6 mapping so the affected ocean and sea-ice area-weighted formulas load areacello from the bundled fx.areacello_ACCESS-ESM.nc resource instead of requesting area_t or t_area as input files. The change covers the ocean global-mean/volume formulas plus the sea-ice area and extent formulas, which now all share the packaged cell-area field already used elsewhere in the model support code.\n\nValidation:\n- pixi run -e dev python -m json.tool src/access_moppy/mappings/ACCESS-ESM1-6_mappings.json\n- Direct JSON assertions that the edited ocean and sea-ice entries no longer reference area_t or t_area

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.7%. Comparing base (42747c9) to head (f2e7928).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #489   +/-   ##
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  Coverage   78.7%   78.7%           
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  Files         33      33           
  Lines       6548    6548           
  Branches    1239    1239           
=====================================
  Hits        5153    5153           
  Misses      1127    1127           
  Partials     268     268           

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