feat(parser): populate inline oneOf discriminator mapping and extract every variant

[gjtorikian]

Summary

Companion to #55 (variant naming). When a oneOf field lists inline anonymous object variants instead of $ref links — the shape that the api_keys spec rename in workos/openapi-spec@058a0a4 introduced for ApiKey.owner and the webhook event payloads — the IR previously dropped the discriminator mapping and let every variant TypeRef collapse to the same qualifyInlineModelName(field, parent). Emitters then had no signal to dispatch on, and the named extracted variants (e.g. UserApiKeyOwner) became orphans in the model registry.

This commit threads the variant names produced by the existing nameVariantModel infrastructure through to the union TypeRef so emitters can render typed dispatch.

• schemas.ts: union construction now resolves a discriminator from three sources, in order: explicit discriminator: with explicit mapping (trust the spec), explicit discriminator: with no mapping (derive from variant const values via deriveInlineVariantMapping), and synthetic detection on inline-only oneOf (detectConstPropertyDiscriminator, then deriveConstNamingDiscriminator + deriveInlineVariantMapping).
• schemas.ts: each oneOf variant's TypeRef is routed through the resolved mapping, producing distinct model refs ({ kind: 'model', name: 'ApiKeyOwner' } for type: organization, { kind: 'model', name: 'UserApiKeyOwner' } for type: user) instead of all collapsing to the parent-derived name.
• extractInlineModelDeep and extractInlineModelsFromProperties: extract every inline variant under a oneOf, not just the first. Without this, only ApiKeyCreatedDataOwner ended up in the model registry and UserApiKeyCreatedDataOwner was missing — emitters would later fail to find the type referenced from the discriminator mapping.

Test plan

• npm test (all 1385 tests pass)
• End-to-end probe via parseSpec(open-api-spec.yaml) against the buggy 058a0a4 spec confirms ApiKey.owner, ApiKeyCreatedData.owner, ApiKeyRevokedData.owner carry typed union variants and a populated discriminator.mapping.
• Both organization and user variant models (ApiKeyOwner / UserApiKeyOwner, ApiKeyCreatedDataOwner / UserApiKeyCreatedDataOwner, ApiKeyRevokedDataOwner / UserApiKeyRevokedDataOwner) now register in spec.models.

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