fix(client): typed multi-response handling with ApiError envelope (closes #8)

[lightsofapollo]

Summary

Closes #8. Operations that declared multiple responses (e.g. 200 + 400) silently collapsed into a single Response type — the inline-naming function in analysis.rs ignored its status_code argument so the second schema overwrote the first. The generated client then tried to deserialize success bodies into the error shape.

This PR reshapes error handling around a typed envelope so the right thing happens whether the success/error bodies are declared, undeclared, or present-but-undeserializable. Discussion of the design is in issue #8.

Design

• ApiError<E> — envelope that always carries status + headers + raw body, with optional typed: Option<E> populated when the body matched a declared schema. Solves the side-tangent ask from Multiple Operation Responses Appear to Not Be Handled #8 (you can now inspect what the server actually sent without modifying the generated code).
• ApiOpError<E> = Transport(HttpError) | Api(ApiError<E>) — new return-type wrapper for every generated operation method.
• Per-operation error enums — operations with declared non-2xx body schemas get {Op}ApiError { Status400(BadRequestBody), … }; ops without declared error bodies fall back to ApiOpError<serde_json::Value> so the body is still inspectable as JSON.
• Always-on raw-body capture — the response handler reads the body to a string before any typed parse, so deserialization failures preserve the bytes.

The reasoning for not using a single all-responses enum is in the issue thread — short version: it forces every caller to match every variant and loses Result ergonomics. Successes are Ok(T), errors are typed Err(ApiOpError<E>).

Breaking changes (0.2.0)

• Generated method signatures change from HttpResult<T> to Result<T, ApiOpError<E>>.
• The HttpError::Http { ... } variant and from_status helper are removed from generated code (replaced by ApiError<E>). The is_client_error / is_server_error helpers move to ApiError<E>.
• Bumped to 0.2.0 per CLAUDE.md "no backwards compat" policy.

Test plan

• cargo test — all suites green (unit + snapshot + integration)
• New end-to-end compile tests in tests/multi_response_client_test.rs:
  • Toy 200+400 multi-response spec — verifies the per-op enum, Status400(BadRequest) variant, and ApiOpError<…> signatures
  • anthropic fixture — full client compiles
  • openai-responses fixture — full client compiles
• Reproducing test from the issue (test_multiple_inline_responses_have_distinct_schemas) — fails on main, passes here
• examples/generated/client.rs regenerated and committed so the example output matches the new shape

🤖 Generated with Claude Code