fix: handle TypeScript Fetch tagged union payloads

[adamcavendish]

Summary

Fixes TypeScript Fetch generation for tagged union payloads and nullable/union-heavy model shapes exposed by a VP-generated client.

What changed

• Promote object-shaped adjacent and external tagged payloads during IR lowering so the TypeScript emitter can inspect variant structure safely.
• Preserve snake_case wire keys while emitting camelCase ergonomic payload keys under property_naming = "camelCase".
• Convert nested tagged payloads recursively in fromJSON and toJSON, including array/map/union wrappers around named models.
• Treat discriminator-only variants as tag-only only when the resolved object is closed; variants with additionalProperties remain contentful.
• Add null guards for nullable required array/ref fields and simplify redundant TypeScript union members.
• Add a masked VP camelCase golden fixture covering tagged payload mapping, tag-only variants, nullable conversion, and union simplification.

Root cause

Tagged-union payload emission assumed payloads were directly named object schemas and could not consistently distinguish closed discriminator-only variants from object-shaped content after lowering. The TypeScript converter path also only applied named-model conversion at the top level, so wrapped named payloads could bypass conversion.

Validation

• UPDATE_GOLDEN=1 cargo test --test golden_tests_typescript_fetch test_toolchain_vp_camel_case_tagged_nullable_unions_golden -- --nocapture
• cargo fmt --all -- --check
• cargo test --test golden_tests_typescript_fetch -- --nocapture
• ./scripts/golden-build-typescript.sh vp
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --quiet
• Downstream VP-generated client check with the fixed local openapi-nexus binary: VP_NODE_VERSION=24.15.0 vp check --no-fmt passed with no warnings, lint errors, or type errors in 869 files.