fix(generator): leave union-declared required names to the variant

[gilesw]

Summary

When an object pairs shared fields with a union, the generator emits a base struct plus a flattened variant. A required name that the shared fields don't declare got a synthesized required serde_json::Value field on the base struct, even when the union's branches declare that name with a real type.

Cloudflare's dns-records_dns-record-response hits this. Its outer required list is name, type, content, data, proxied, ttl, settings, comment, tags. All of those are declared only inside the per-record-type branches, and data exists only on CAA, SRV, HTTPS and similar records. The result:

• every A, AAAA, CNAME, MX, NS, PTR, TXT and OPENPGPKEY record fails to deserialize with missing field `data`
• name, content, ttl, type and the other fields are typed serde_json::Value on the base struct, even though the branch structs already carry them typed (for example content: Option<std::net::Ipv4Addr> on ARecord)

The bug occurs in both forms the schema takes: the current spec uses allOf: [anyOf: [...], {properties}], and the corpus-pinned spec puts properties and anyOf side by side.

This PR leaves a required name to the variant when any branch reachable through the union declares it. The branch types the field and keeps its own optionality. A required name that no shared member or branch declares still gets the existing placeholder.

Generated compatibility

• Generated model or method signatures: for affected schemas, the base struct loses the untyped fields that the variant already carries. In the corpus this is only Cloudflare: DnsRecordsDnsRecordResponse and four DNS operation Response4xxResult types that embed it. types.rs shrinks by 210 lines. No other spec changes.
• Query/path/header/body wire behavior: inputs that decoded before still decode, and serialization emits the same keys. Inputs that omit a branch-only field, such as data on an A record, now decode.
• Generated runtime dependencies or features: none.
• Configuration defaults or migrations: none.
• Remaining unsupported OpenAPI shapes: none introduced.

Validation

• Added a reduced Cloudflare-shaped fixture to tests/allof_object_union_preservation_test.rs in both forms: RecordResponse (allOf) and SiblingRecordResponse (sibling properties + anyOf). It checks that the base struct no longer owns the branch-declared names, and round-trips A, MX and CAA records through a compiled scratch crate. Both tests fail without the change with missing field `data` .
• cargo fmt --check
• cargo clippy --all-features -- -D warnings
• cargo nextest run --all-features --no-fail-fast: 726 run, all pass. In an earlier run, required_undeclared_members_round_trip_untyped_and_typed_values failed while other cargo builds were running on the machine. It passed when rerun alone and in the full rerun.
• cargo test --doc --all-features and cargo doc --no-deps --all-features with -Dwarnings
• scripts/spec-compile.sh anthropic openai: both PASS + ROUNDTRIP
• Refreshed tests/corpus-manifest.txt; --check is clean
• Changelog entry under Unreleased

Notes for reviewers

• Smallest review path: union_declared_property_names and its two call sites in src/analysis.rs: the reconciliation at the end of analyze_allof_composition, and the end of analyze_object_with_variants. Then the new fixture.
• Lazy analysis: the helper analyzes a union's $ref branch targets when they aren't cached yet, the same way merge_analyzed_object_properties resolves references. Without that, $ref-only unions report no declared names.
• Base catch-all map: in the sibling form, the base had switched to an untyped additional_properties map only to hold the placeholders. Once those placeholders go, it returns to closed, unless the schema declares additionalProperties or its examples or minProperties/maxProperties need the map.
• Found while evaluating the generator for a Rust Cloudflare CLI, the same work that led to fix(generator): accept wire-equivalent encodings when selecting a union branch #82. With this change, a zones and DNS subset of the current Cloudflare spec decodes list responses into DnsRecordsARecord, DnsRecordsMXRecord and so on, without patching the spec.

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