fix(pinia-orm): scope decorator mutators, casts & hidden fields per entity

[CodeDredd]

Bug

fieldMutators, fieldCasts and the hidden array live as shared static objects on the base Model class. setMutator(), setCast() and setHidden() (used by the @Mutate, @Cast and @Hidden decorators) wrote into these shared objects keyed only by field name — so a mutator/cast/hidden registered for one entity leaked to every other entity with a same-named field.

• Decorator mutators for properties with same name on different entities are mixed up #1937: decorator mutators for firstName on two entities — only one runs, for both entities
• When adding a mutation dynamically via "setMutator", all fields with the same name across all stores have the mutation applied #1992: setMutator() on one store mutates all same-named fields in every store

Fix

Key fieldMutators and fieldCasts by modelEntity() — the same pattern schemas, registries and fieldsOnDelete already use. setHidden() now does copy-on-write so it no longer pushes into the base class' shared array.

Tests

4 new regression tests (mutator decorator, setMutator, cast decorator, hidden decorator across two entities with same-named fields). All fail before the fix, pass after; full suite green (401 tests).

fixes #1937
fixes #1992

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.50%. Comparing base (a7cc431) to head (dda53b4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2027      +/-   ##
==========================================
+ Coverage   99.43%   99.50%   +0.06%     
==========================================
  Files          82       82              
  Lines        3019     3022       +3     
  Branches      529      530       +1     
==========================================
+ Hits         3002     3007       +5     
+ Misses         14       12       -2     
  Partials        3        3              

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