fix(codegen): preserve asset_type discriminator on Individual*Asset slot types (#1498)

[bokelley]

Closes #1498.

Why

json-schema-to-typescript was flattening the schema's allOf:[baseIndividualAsset] + properties.asset_type.const discriminator on the 14 IndividualXAsset slot types, leaving them as bare BaseIndividualAsset aliases. Adopters writing TS-clean code constructing one of these without asset_type got runtime VALIDATION_ERROR against the wire schema with no compile-time signal — exactly the class of codegen mismatch we've burned down before (date vs date-time, Foo/Foo1 dedupe).

What

• New applyIndividualAssetDiscriminators post-processor in scripts/generate-types.ts.
• Rewrites all 14 Individual*Asset = BaseIndividualAsset aliases into discriminated intersections:

export type IndividualImageAsset = BaseIndividualAsset & {
  asset_type: 'image';
  requirements?: ImageAssetRequirements;
};

• Applied to both core.generated.ts and tools.generated.ts. The requirements?: field is conditionally emitted — only when the per-type *AssetRequirements interface is declared in the same file (avoids TS2304 in tools.generated.ts where these aren't redeclared).
• Regenerates core.generated.ts, tools.generated.ts, and the downstream schemas.generated.ts (Zod schemas auto-derived from TS types).

Compat

Strictly additive at runtime — wire validator already rejected these. The new TS errors catch a pre-existing bug at compile time. Adopters who construct IndividualImageAsset literals without asset_type will see TS errors that tell them what they were already getting at runtime.

Test plan

• npm run format:check — clean
• npm run typecheck — clean
• npm run build:lib — clean
• node --test test/examples/hello-creative-adapter-template.test.js — 3/3 pass (the gate most likely to use these types)
• Verified discriminator catches missing asset_type via standalone TS test:

  src/lib/types/core.generated.ts: IndividualImageAsset literal without asset_type
  → TS2322: Property 'asset_type' is missing
  

• Full unit-test suite: 5960 pass, 69 fail; baseline (pre-change) was 5883 pass / 66 fail / 2 cancelled. Failures are pre-existing (AgentClient context-management flakes, unrelated to codegen).

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