[adamcavendish]

Summary

Preserve explicit additional properties in Python httpx and requests clients,
including both pure dictionaries and objects with named fields.

Problem

The IR already retains dictionary value facts, but Python object emission used
only named properties. A mixed-object round-trip therefore silently discarded
every extension entry. Pure dictionaries became empty dataclasses without normal
from_dict / to_dict conversion.

Behavior

Models now expose typed, mutable, keyword-only additional_properties storage
with an independent empty-dict default. Conversion separates known wire fields
from extension entries and writes them back as one flat JSON object.

value = MixedMap(
    known_field="fixed",
    additional_properties={"keep_key": NestedValue(wire_value="nested")},
)
value.to_dict()  # {"known_field": "fixed", "keep_key": {"wire_value": "nested"}}

Situation	Behavior
Pure dictionary	Normal dataclass storage and conversion, not an empty model
Opaque extension values	Keep JSON trees, null entries and large integers
Typed extension values	Use declared model, enum, alias and container codecs
Renamed-looking keys	Stay verbatim; only exact declared wire keys are separated
Storage-name collision	Append underscores to storage; keep authored fields intact
Declared wire-key collision	Raise ValueError, even when the named field is omitted

Aliases, recursive dictionaries, containing models and union boundaries retain
the same typed value conversions. Dictionary models participate in outbound
matching through their declared storage, without invoking codecs to count union
candidates. Existing firstMatch, strict and raw policies still govern union
values. HTTP JSON requests, successful responses and lazy typed error bodies use
the same codecs, including native decode failures and error caching.

Factory and typing helper references remain distinct from authored names such as
field, dict, additionalProperties and Any. Factory type references are
deferred so recursive model imports stay usable.

Boundaries

This covers both Python transports only. Existing named constructor parameters,
closed/implicit objects and ordinary field codecs remain intact; extension
storage is an additive keyword-only field. Existing tagged helper keyword
parameters and ordinary optional/nullable field omission remain unchanged.
Generated source and dependency manifests are consumed without compatibility
patches.

Shared IR/lowering, other-language generators, multi-member allOf composition
and tagged-wrapper extension storage are not redesigned. General inline composed
additional-property schemas that already lower as opaque values remain opaque;
that existing loss of type facts is a separate lowering repair. This change
converts the retained value facts, not a new JSON Schema validation policy.

[codecov]

Codecov Report

❌ Patch coverage is 98.95833% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/generators/python/additional_properties.rs	98.70%	1 Missing ⚠️
src/generators/python/value.rs	97.50%	1 Missing ⚠️

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