feat(orm): implement discriminated union return types for delegate (polymorphic) models

[Copilot]

Summary

Querying a delegate (polymorphic base) model such as db.asset.findMany() now returns a proper TypeScript discriminated union instead of the flat base type.

Before: findFirstOrThrow() returned Asset — accessing r.duration was always a type error, and narrowing by r.assetType had no effect on the type.

After: findFirstOrThrow() returns:

| (ImageFields   & { assetType: 'Image' })
| (RatedVideoFields & { assetType: 'Video'; videoType: 'RatedVideo' })

TypeScript can now narrow the result via the discriminator field:

const r = await db.asset.findFirstOrThrow();
if (r.assetType === 'Video') {
    console.log(r.duration); // ✅ Video-specific field
    console.log(r.rating);   // ✅ RatedVideo-specific field (only submodel)
} else {
    console.log(r.format);   // ✅ Image-specific field
}

Multi-level delegation (Asset → Video → RatedVideo) is fully expanded so every concrete leaf model appears in the union. The select path continues to return a flat result as before (explicitly selected fields remove polymorphic information).

Changes

packages/orm/src/client/crud-types.ts

• Added IsDelegateModel, GetModelDiscriminator, GetSubModels to schema imports.
• Added FlatModelResult helper type — the original scalar-field object expression extracted into a named alias for the leaf case.
• Added DelegateUnionResult<Schema, Model, Options, SubModel, Omit, _Depth>:
  • Distributes TypeScript's union over each direct sub-model.
  • When a sub-model is itself a delegate, recurses into its sub-models so all concrete leaf types appear with all ancestor discriminator fields fixed to literal values.
  • Uses a tuple-based depth counter (hard stop at 10) as an additional safety net.
• Modified DefaultModelResult to use DelegateUnionResult for delegate models.
  • Added _IsGenericModel default type parameter (checks [string] extends [Model]) that short-circuits the entire delegate expansion when TypeScript is evaluating ClientContract<SchemaDef> (the generic base interface). This keeps the type instantiation count bounded (~1.9M vs 1.6M baseline, well below TypeScript's recursion limit).

tests/e2e/orm/schemas/delegate/typecheck.ts

• Uncommented the two discriminated-union narrowing test blocks (for findFirstOrThrow and the include: { assets: true } relation path).