fix(openapi): handle discriminator mapping without oneOf in both conversion paths

[thesandlord]

Description

Refs: Slack thread from Sandeep Dinesh

Link to Devin run: https://app.devin.ai/sessions/eddb3e06655c47ec89a88b5bced08a4e
Requested by: Sandeep Dinesh (@thesandlord)

Fixes handling of OpenAPI schemas that have discriminator.mapping but no explicit oneOf/anyOf. This pattern is used by some APIs (like Dropbox Sign) where a base schema defines a discriminator with mapping to variant schemas, but doesn't list them in a oneOf.

Previously, such schemas were incorrectly converted as plain objects. Now they are properly converted as discriminated unions.

Changes Made

• Added tryConvertDiscriminatorMappingSchema() method in SchemaConverter.ts (for --from-openapi direct path) that:
  • Detects schemas with discriminator.mapping but no oneOf/anyOf
  • Synthesizes a oneOf from the mapping values
  • Delegates to OneOfSchemaConverter for proper discriminated union conversion
• Added handling in convertSchemas.ts (for OpenAPI→Fern conversion path) that:
  • Detects the same discriminator.mapping pattern
  • Calls convertDiscriminatedOneOf which uses the mapping internally to build union subtypes
• Added test fixture discriminator-base-class demonstrating this pattern
• Bumped CLI version to 3.33.0

Updates since last revision

• Rebased on latest main
• Removed smoke test API changes (per user request)
• Added version bump to 3.33.0 in versions.yml

Testing

• Unit tests added/updated (new test fixture with snapshots)
• Manual testing completed (lint checks pass)
• Verified fix works on Dropbox Sign docs preview (discriminated union now renders with "Hide 10 variants" UI)

Human Review Checklist

• Verify the fix handles edge cases where discriminator.mapping exists but the schema should remain an object
• Confirm both code paths (SchemaConverter.ts and convertSchemas.ts) produce consistent results
• Note: The openapi-ir-parser path calls convertDiscriminatedOneOf directly without synthesizing oneOf first - verify this is correct since convertDiscriminatedOneOf uses discriminator.mapping internally (line 51-74 of convertDiscriminatedOneOf.ts)

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[thesandlord]