Fix two unmasked bugs in oceanBgchem derivations

[rhaegar325]

Bugs

1. ph — KeyError: 'log10'

The ph mapping uses {"operation": "log10", "args": ["htotal"]}, but log10
was never registered in custom_functions.

2. intdic / intppnitrate — KeyError: 'st_ocean'

evaluate_expression evaluates kwargs values recursively, and a bare string
is a variable lookup (context[expr]). So "kwargs": {"dim": "st_ocean"} looks
for a model variable named st_ocean instead of passing the dimension name.
Constants must be wrapped as {"literal": "..."} — the convention already used
by ocontempmint, somint, mrso and mrfso.

Four entries were written with the bare form before that convention existed:
ocean.hfsifrazil2d, intdic, intpoc, intppnitrate (in both ESM1-5 and
ESM1-6).

Changes

• Register log10 in custom_functions.
• Wrap all 8 bare-string dim kwargs as {"literal": "st_ocean"}.
• Raise a KeyError that names the available model variables and suggests the
  {"literal": ...} form, instead of the bare 'st_ocean'.
• Add TestMappingKwargsAreResolvable: every string-valued kwarg in every
  mapping file must name a model variable or a registered function.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.7%. Comparing base (becf94e) to head (05842fa).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #729   +/-   ##
=====================================
  Coverage   79.7%   79.7%           
=====================================
  Files         41      41           
  Lines       9162    9167    +5     
  Branches    1707    1707           
=====================================
+ Hits        7301    7306    +5     
  Misses      1526    1526           
  Partials     335     335           

Flag	Coverage Δ
unit	79.7% <100.0%> (+<0.1%)	⬆️

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