feat: preserve generated response headers

[adamcavendish]

Summary

Preserves response headers consistently across every generated client and adds optional typed accessors for OpenAPI-declared headers.

Problem

Successful responses did not expose headers consistently, Rust error handling converted headers through a lossy string vector, and referenced response-header components were dropped during lowering. This made headers such as Retry-After unavailable or adapter-dependent.

Behavior

Situation	Behavior
Successful or HTTP error response	Retains the backend-native header collection
Undeclared header	Remains available through the native/raw response
Declared string header	Optional string accessor
Declared integer, number, or boolean header	Optional typed accessor using canonical scalar wire syntax
Missing or malformed declared header	Accessor returns its empty result without failing response handling
Colliding normalized header names	Generates deterministic, collision-safe accessor names
Referenced or cyclic component header	Resolves valid references and returns a lowering error for cycles

Compatibility

Rust generated response structs now include a native headers field, and ApiError::headers() returns the backend-native HeaderMap instead of Vec<(String, String)>. Python retains existing body-only methods and adds collision-safe *_with_http_info() methods. Browser Fetch visibility still follows CORS Access-Control-Expose-Headers.

No package version or release metadata changes are included.

Validation

• cargo fmt --all -- --check
• just clippy
• just doc-check
• just book
• just test-all
• just golden-check
• just golden-build-all

[codecov]

Codecov Report

❌ Patch coverage is 96.19687% with 51 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/generators/response_headers.rs	78.22%	27 Missing ⚠️
src/ir/lower/v30.rs	94.28%	6 Missing ⚠️
src/ir/lower/v31.rs	94.28%	6 Missing ⚠️
src/ir/lower/v32.rs	94.28%	6 Missing ⚠️
src/generators/typescript/fetch/sigil_emit_api.rs	96.92%	4 Missing ⚠️
src/generators/python/operation_names.rs	95.12%	2 Missing ⚠️

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