Restore tob Kelvin conversion

[rbeucher]

Summary

• restore the kelvin_to_celsius wrapper for the ACCESS-ESM1-6 tob mapping
• keep tob as kelvin_to_celsius(ocean_floor(pot_temp))
• retain the guarded Kelvin conversion fallback used by the helper

Validation

• pixi run -e dev python -m pytest tests/unit/test_derivations.py -q
• pixi run -e dev python -m coverage run -m pytest tests/unit/test_derivations.py -q && pixi run -e dev python -m coverage report -m src/access_moppy/derivations/__init__.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.7%. Comparing base (b5e3948) to head (959e86c).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #488     +/-   ##
=======================================
+ Coverage   78.6%   78.7%   +0.1%     
=======================================
  Files         33      33             
  Lines       6534    6548     +14     
  Branches    1235    1239      +4     
=======================================
+ Hits        5135    5153     +18     
+ Misses      1129    1127      -2     
+ Partials     270     268      -2     

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

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