feat(pinia-orm): add updateOrCreate & firstOrCreate repository methods

[CodeDredd]

Feature (#1833)

Laravel-style upsert helpers on the repository:

// return the first user named Jane, or create her with age 40
const user = useRepo(User).firstOrCreate({ name: 'Jane Doe' }, { age: 40 })

// update Jane's age, or create her with the merged attributes
useRepo(User).updateOrCreate({ name: 'Jane Doe' }, { age: 41 })

• Matching compares every attribute pair with strict equality via a where-closure.
• updateOrCreate updates through save() using the matched record's primary key (composite keys supported).
• Docs: new Update Or Create section on the Updating Data guide page.

Tests

7 new tests: create-when-missing, update-when-matching, multi-attribute matching, composite primary keys, uid generation, firstOrCreate both paths. Full suite green.

closes #1833

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.40%. Comparing base (018acee) to head (80b1c12).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2032   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files          82       82           
  Lines        3025     3042   +17     
  Branches      533      537    +4     
=======================================
+ Hits         3007     3024   +17     
  Misses         15       15           
  Partials        3        3           

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