feat(pinia-orm): hydrate recursively discriminated models

[CodeDredd]

Feature (#1995)

STI only resolved one discriminator level: a record for Document -> File -> Video was hydrated as File, never as Video.

• New Model#$getDiscriminatedModel(record) walks nested type keys (each level may define its own typeKey + types()) and returns the most specific matching model. Cycle-safe via a visited set (the self-referencing entries in inherited types() maps terminate the walk).
• Query#getHydratedModel() (find/all/save hydration) and Repository#make() both use it — make() previously did no type resolution at all.
• useRepo() now registers nested type models to the database recursively, so saving into deep hierarchies resolves their entities.
• Docs: new Nested Discriminators section on the STI page.

Tests

New STI test with a 3-level hierarchy (Animal -> Dog -> Terrier, second discriminator race): make() resolves to the deepest type incl. its fields, intermediate and base types still resolve correctly, and find() after save() returns the specific type. Full suite green (406 tests).

closes #1995

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.47%. Comparing base (c6bebb9) to head (86a6a80).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2037      +/-   ##
==========================================
+ Coverage   99.40%   99.47%   +0.06%     
==========================================
  Files          82       82              
  Lines        3025     3046      +21     
  Branches      533      538       +5     
==========================================
+ Hits         3007     3030      +23     
+ Misses         15       13       -2     
  Partials        3        3              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.