[adamcavendish]

Summary

Preserve explicit additional properties in Java OkHttp models, including pure
dictionaries and objects with named fields.

Problem

The IR retains additional-value types, but Java object emission used only named
properties. Gson therefore silently dropped mixed-object extensions and turned
pure dictionaries into empty objects.

Behavior

Models expose typed, mutable getAdditionalProperties() storage with an independent
empty-map default. Existing named constructors and getters stay unchanged.

MixedMap value = new MixedMap("fixed");
value.getAdditionalProperties().put("keep_key", new NestedValue("nested"));
// Gson writes {"known_field":"fixed","keep_key":{"wire_value":"nested"}}

Situation	Behavior
Pure dictionary	Normal model with flat entries, not a Map subtype
Opaque values	JsonElement preserves trees, nulls and numeric precision
Typed values	Declared Gson codecs retain models, aliases and policies
Extension keys	Stay verbatim rather than following Java field renaming
Storage collision	Numeric suffix keeps authored fields and getters intact
Declared-key clash	IllegalArgumentException, even for omitted named fields

Configured Gson delegates still own ordinary fields. Native streaming/tree
integer behavior, custom field/value adapters, configured numeric writing and
ordinary null omission remain intact. Extension null preservation restores the
surrounding writer setting even on failure. HTTP JSON requests, successful
responses and lazy typed-error bodies retain their dictionary entries.

Boundaries

Java OkHttp only. Closed/implicit models, alias wrappers, generated dependency
versions, shared lowering and other-language generators are unchanged. General
inline union type loss, primitive-union helper-name collisions, multi-member
allOf and tagged-wrapper extension storage remain separate repairs.

The support uses tree-reader/writer implementation classes from the unchanged
Gson 2.11.0 dependency to retain its native input/output behavior. Native consumers
cover both tree and streaming entrypoints. Ordinary fields are buffered for their
configured delegate rather than reimplemented as a new validation policy.

Generation formatting remains a separate recorded follow-up; this PR does not
add a formatter or claim generated-source formatter conformance.

[codecov]

Codecov Report

❌ Patch coverage is 97.05882% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/generators/java/okhttp/additional_properties.rs	96.57%	6 Missing ⚠️

📢 Thoughts on this report? Let us know!