Give lev_bnds its own formula_terms from z_bounds_factors (CF §4.3.3)

[rhaegar325]

A parametric vertical coordinate's bounds variable needs its own formula_terms, referencing the bounds of each term. For hybrid-height variables (cl, cli, clw) lev_bnds carried none, failing the CF-1.11 check:

§4.3.2: lev_bnds's formula_terms is a required attribute
        and must be a non-empty string

lev itself gets formula_terms by inheritance from the UM source coordinate (theta_level_height), retargeted to the post-rename names. Its bounds variable has no attributes in the source, and the CF §7.1 loop in update_attributes() replaces bounds attrs wholesale from an INHERIT_KEYS subset that never included formula_terms.

The correct value was already in the coordinate table as z_bounds_factors ("a: lev_bnds b: b_bnds orog: orog"), until now read only to decide which bounds variables to create. Apply it after the §7.1 loop, which would otherwise wipe it. Adding formula_terms to INHERIT_KEYS would instead copy the parent's "a: lev b: b orog: orog", which points at the coordinates rather than their bounds and is equally non-compliant.

Skipped when z_bounds_factors is empty (hybrid_height_half) or when any referenced variable is absent, so no dangling reference is written.

Verified end to end on cl: §4.3 Vertical Coordinate goes from [5, 6] with the message above to [10, 10] clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.2%. Comparing base (3ccc6ca) to head (094a53d).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #641   +/-   ##
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  Coverage   77.2%   77.2%           
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  Files         40      40           
  Lines       8514    8522    +8     
  Branches    1590    1593    +3     
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+ Hits        6575    6583    +8     
  Misses      1610    1610           
  Partials     329     329           

Flag	Coverage Δ
unit	77.2% <100.0%> (+<0.1%)	⬆️

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[rbeucher]

Fixes #642