Fix TypeScript Fetch tagged oneOf union member conversion

[adamcavendish]

Problem

typescript-fetch generated correct camelCase-to-wire converters for each member of a tagged oneOf, but the union converter itself used an unchecked cast. This happened when the tagged oneOf members were plain inline objects whose discriminator is a required single-value string enum alongside two or more content properties.

TaggedEnumPattern only recognized internally tagged oneOf members wrapped in allOf, so these plain inline members fell back to an untagged Union. TypeScript camelCase mode emits cast-through converters for untagged unions, so nested request and response models kept the wrong property spelling.

Fix

• Detect plain internally tagged oneOf variants for OAS 3.0, 3.1, and 3.2:
  • object has more than two properties
  • no allOf
  • has a required property whose schema is a single-value string enum (the discriminator)
• Add lower-level regression tests for all three OAS versions.
• Add a TypeScript camelCase golden fixture that exercises the union converters through a nested request and response model.

Validation

• cargo fmt --check passes
• cargo clippy --lib passes
• cargo test --lib passes (665 tests)
• cargo test --test golden_tests_typescript_fetch test_property_naming_camel_case_tagged_union_plain_internal_golden passes

[codecov]

Codecov Report

❌ Patch coverage is 95.54455% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/ir/tagged_enum_pattern.rs	91.30%	6 Missing ⚠️
src/ir/lower/v30.rs	97.56%	1 Missing ⚠️
src/ir/lower/v31.rs	98.03%	1 Missing ⚠️
src/ir/lower/v32.rs	97.56%	1 Missing ⚠️

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