spec(schemas): add discriminator hints to const-property oneOf unions (adcp#3917)

[bokelley]

Summary

Step 2 of the discriminator audit plan from #3917. Adds discriminator: { propertyName } to 16 oneOf unions in static/schemas/source/ whose variants already declare the same required property as a const with distinct string values. Also tightens the audit walker to enforce that any discriminator.propertyName=X is backed by every non-ref variant declaring properties.X as required const — preventing a two-sources-of-truth drift where the keyword sits without the structural narrowing.

Non-breaking — the OpenAPI discriminator keyword is ignored by JSON Schema 2020-12 validators that don't recognize it. Existing const-property narrowing remains the source of truth. Codegen targets that respect the keyword (msgspec, openapi-typescript, datamodel-code-generator) now emit a properly-narrowed union without per-variant casts.

Affected schemas (16 unions, 15 hoists across 12 files)

• adagents.json (authorization_type)
• compliance/comply-test-controller-response.json (attestation_mode)
• content-standards/artifact.json (type, method)
• core/activation-key.json (type)
• core/creative-item.json (asset_kind)
• core/deployment.json (type)
• core/destination.json (type)
• core/optimization-goal.json (kind × 3)
• core/requirements/catalog-field-binding.json (kind × 2)
• core/signal-pricing.json (model)
• creative/preview-creative-response.json (response_type)
• creative/preview-render.json (output_format)

Walker tightening

scripts/audit-oneof.mjs now treats discriminator.propertyName=X as ✗ if any non-ref variant fails to declare properties.X as required const, or if two variants share the same const value. Nested-union and pure-ref variants are trusted (their target may carry the const further down). This guards the failure mode the protocol expert flagged: a future author could add discriminator.propertyName=X without the const, weakening validation for non-OpenAPI consumers.

Deferred to follow-ups

• Boolean discriminators (get-adcp-capabilities-response.json supported, update-content-standards-response.json success) — Ajv requires unique-string values
• Cross-file ref-only variants (core/format.json assets/items → asset_type, core/pricing-option.json → pricing_model) — Ajv discriminator support doesn't follow cross-file $ref. (Within-file #/definitions refs work fine and are already deployed in catalog-field-binding.json.)

Test plan

• node scripts/audit-oneof.mjs — 36 ✓ unchanged (these were already classified ✓ via const)
• Walker drift test: temporarily setting discriminator.propertyName="wrong_key" on core/activation-key.json correctly trips ✗ with a clear "missing const+required" message
• Pre-existing core/assets/asset-union.json and core/offering-asset-group.json (which use nested-union ref variants) still pass — the invariant trusts nested-union targets
• npm run test:json-schema (257/0)
• npm run test:schemas (7/0)
• npm run test:composed (26/0)
• npm run test:oneof-discriminators — passes against committed baseline

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