[adamcavendish]

Summary

Preserve Kotlin OkHttp untagged-union member payloads with enum-aware structural
selection and configurable typed or raw adapters.

Problem

The existing Gson adapter chooses object members by required keys alone. Members
with identical required keys but disjoint numeric enums therefore select the first
variant and silently discard fields from the actual matching member. The same
failure affects containing fields, lists, and maps.

Kotlin sealed subclasses also need serialization through their declared member
type: reflecting a nested union's runtime variant emits a value0 wrapper instead
of that union's wire payload.

Behavior

Policy	Result
firstMatch	Default: first structural match in schema order, before opaque fallbacks.
strict	Reject multiple authored concrete matches before decoding one member.
raw	Explicit VariantRaw(JsonElement) preserving known and unknown JSON.

Matching retains finite scalar constraints, nested arrays/maps, null permissions,
and productive recursion. Exact decimal significands and unbounded exponents avoid
floating-point rounding and matcher-only scale limits. Recursive checks cache both
successful and failed predicates by JSON-node identity.

Typed adapters delegate once to the selected original member. Nested strict and
native decode errors propagate without retrying another member or raw fallback.
Serialization uses the declared payload type to preserve nested union boundaries.
Raw adapters preserve null object entries and large numbers while restoring the
writer's previous null policy, including on failure.

Compatibility

• Default and strict modes retain sealed classes, variant names, typed payloads,
  and existing root-null behavior (null or nullable-union VariantNull).
• Kotlin typealias declarations remain transparent; intersections retain their
  existing wrapper fields rather than receiving a new allOf wire codec.
• Ordinary models, enum declarations, tagged codecs, generated dependencies,
  shared IR, and other-language generators are unchanged.
• Raw mode is opt-in through kotlin-okhttp.untagged_union_strategy; an enclosing
  VariantRaw model uses VariantRawVariant for its raw case.

Boundaries

This is the Kotlin-only union repair following merged Java #175. Dictionary-only
models, additional-property preservation, and multi-member allOf redesign remain
separate work. Structural selection is not full JSON Schema validation; format
bounds and existing native codec limits remain with the selected member's codec.

[codecov]

Codecov Report

❌ Patch coverage is 89.57597% with 59 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/generators/kotlin/okhttp/matching.rs	85.55%	51 Missing ⚠️
src/generators/kotlin/okhttp/union.rs	95.16%	6 Missing ⚠️
src/generators/kotlin/okhttp/sigil_emit.rs	95.45%	2 Missing ⚠️

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