fix(pinia-orm): accept eager load callback for optional relations

[CodeDredd]

Bug

When a relation is declared as an optional property —

@HasMany(() => Review, 'shipment_id') declare reviews?: Review[]

— the property type includes undefined, so WithKeys<M> skipped the key and the callback conditional in with() / whereHas() / whereDoesntHave() fell through to () => void / never. Passing a query constraint callback then failed to compile (#1999).

Fix

• WithKeys<T> now strips optionality (-?) and checks NonNullable<T[P]>
• the callback conditionals additionally accept undefined in the union (M[T] extends Model | Model[] | null | undefined ? …)
• same treatment for the exported UltimateKeys helper

Type-only change, no runtime behavior touched.

Tests

New test using an optional comments?: Comment[] relation with a with() constraint callback — produces 2 tsc errors before the fix, compiles clean after (verified with tsc --noEmit; no new errors elsewhere). Full suite green (402 tests).

fixes #1999

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.43%. Comparing base (7a32aec) to head (a5812ac).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2030   +/-   ##
=======================================
  Coverage   99.43%   99.43%           
=======================================
  Files          82       82           
  Lines        3022     3022           
  Branches      530      530           
=======================================
  Hits         3005     3005           
  Misses         14       14           
  Partials        3        3           

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