[adamcavendish]

Summary

Fix enum-aware untagged-union member selection in all three Rust backends:
reqwest, ureq, and aioduct. Correctly select the declared member and retain its
payload through properties, containers, recursive models, and HTTP responses.

Problem

Serde's derived untagged dispatch tries members whose constrained numeric fields
are still ordinary Rust integers. For a union of revision: enum [1] and
revision: enum [2], {"revision":2,"second_value":"two"} previously selected the
first member and re-encoded as {"revision":2}. Direct member decoding worked;
the union silently discarded the actual member's payload.

Behavior

Policy	Result
firstMatch (default)	Keep typed variants; choose the first structural match, concrete before fallback.
strict	Keep typed variants; reject multiple concrete candidates before decoding.
raw	Emit only Raw(serde_json::Value) and preserve known, unmatched, or null JSON.
No typed match	Return a deserialization error without adding a public fallback variant.
Selected decoding fails	Propagate the original error, including nested strict-union errors.

Matching consumes shared IR facts: required wire keys, scalar finite constraints,
nested arrays/maps, null boundaries, and productive recursive references. Numeric
enum comparisons retain integer precision above 2^53. Recursive checks share
shape/node results within one selection, avoiding exponential subtree scans
without changing candidate order or sharing cached results across decoded values.

Common union derives and per-type overrides accept equivalent short,
crate-qualified, and absolute Serde/utoipa paths without duplicating owned
implementations. Tagged model emission failures preserve the underlying
diagnostic and model context rather than silently omitting the model.

Compatibility and boundaries

• Default/strict typed variants, payload types, and untagged serialization remain
  unchanged. Deserialization no longer accepts a member whose retained enum or
  structural constraints do not match. Explicit raw mode changes the union API.
• Structural selection is not full JSON Schema validation. Native Serde decoding
  still applies after selection; ordinary model serialization/validation is not
  redesigned. The shared Rust generator config gains untagged_union_strategy.
• No synthetic Null or Raw variant is added to typed enums. Explicit nullable
  use sites retain Option<Union>; named root-nullable union representation and
  automatic reference nullability are unchanged.
• Unknown-field retention, dictionary/additional-property repairs, and
  multi-member allOf redesign remain separate. Use raw mode to retain arbitrary
  union JSON rather than relying on a typed member to preserve unknown fields.
• Only Rust generator behavior changes. The Python pattern merely ignores the
  added shared-matcher metadata; Java, Kotlin, Go, and TypeScript are unchanged.

[codecov]

Codecov Report

❌ Patch coverage is 87.15891% with 80 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/generators/rust/common/matching.rs	84.25%	54 Missing ⚠️
src/generators/rust/common/emit_models.rs	74.24%	17 Missing ⚠️
src/generators/rust/common/union.rs	95.10%	9 Missing ⚠️

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