[adamcavendish]

Summary

Convert Python untagged-union members consistently in both httpx and requests
clients, preserving their existing dataclass and API names.

Problem

The generated union was only a type alias. Its containing models and HTTP methods
treated members as identity values: typed requests failed with Object of type ClientTool is not JSON serializable, while decoded responses retained dictionaries
instead of the declared dataclasses. Finite enum constraints also need to select
the actual member rather than a different object with the same required keys.

Behavior

Policy	Selection	Unmatched value
firstMatch	First concrete match in schema order	Preserve the original value
strict	Reject multiple original concrete matches	Use an opaque fallback or error
raw	Keep union subtrees dynamic and unconverted	Preserve the original value

firstMatch is the default. Concrete candidates precede opaque fallbacks. Matching
retains finite values, Python boolean/number distinctions, nullable boundaries,
and productive recursion. Delegation occurs after selection, so nested strict
errors are not swallowed by an outer alternative or nullable wrapper.

The shared Python path covers model fields, aliases, nested arrays/maps, JSON and
multipart payloads, success responses, tagged payload delegates, and lazy cached
error bodies. Required union bodies containing None are sent as JSON null.
Both transports have native consumers and strict public-declaration contracts.

Compatibility

Existing constructors, method names, and public union aliases remain available.
Named union modules add *_from_dict / *_to_dict helpers without adding internal
helpers to package barrels. raw deliberately changes a union subtree's public
type to object; callers supply JSON-compatible values themselves. Optional
request bodies keep the existing convention that None means omit.

This PR is Python-only. Other languages' emitters and output remain unchanged.
Dictionary/additional-property preservation and multi-member allOf modeling
remain separate repairs; these matching policies are not full schema validation.

[codecov]

Codecov Report

❌ Patch coverage is 88.46329% with 253 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/generators/python/emit_models.rs	84.67%	165 Missing ⚠️
src/generators/python/value.rs	90.20%	71 Missing ⚠️
src/generators/python/httpx/emit_api.rs	95.65%	6 Missing ⚠️
src/generators/python/requests/emit_api.rs	95.65%	6 Missing ⚠️
src/generators/python/emit_api_values.rs	95.04%	5 Missing ⚠️

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