fix(anthropic): return API error for malformed non-streaming responses

[leseb]

Summary

The Anthropic-to-OpenAI filter rewrites response headers to advertise an Anthropic response contract, but an upstream can return HTTP 200 with a body that cannot be transformed (malformed JSON, empty, or missing) — previously that broken body was forwarded unchanged, so clients received a response that did not conform to the advertised contract. transform_non_streaming_body now replaces any untransformable success body with a schema-complete Anthropic api_error envelope and preserves the upstream request id (now captured on the success path too). This is the smallest complete change: it only rewrites the body on the failure-to-transform path and leaves successful translations untouched.

Related issue

Closes #835

Validation

• Unit tests — cargo test -p praxis-ai-apis --lib anthropic (206 passed), covering missing / empty / malformed non-streaming success bodies
• Integration or functional tests — anthropic_to_openai_replaces_malformed_success_body proxy test and all_inference_fixtures_replay with the new synthetic messages/malformed-success recording
• make lint (clippy, rustfmt, deps, docs, examples, check-inference, README sync) and make build

Checklist

• I reviewed every changed line and can explain the change.
• New capabilities include an example config and functional example test. — bug fix; adds a functional example test against the existing anthropic/messages-to-openai.yaml config.
• User-facing behavior and generated documentation are updated. — generated inference coverage README regenerated via sync-inference-readme.
• Performance-sensitive changes include appropriate benchmark or load-test evidence. — N/A; only the failure-to-transform path is affected.
• Commits are signed and include a Signed-off-by trailer.

Breaking changes

None. Only responses that previously could not be transformed change behavior: instead of a non-conformant body forwarded under Anthropic headers, clients now receive a valid Anthropic api_error envelope. (The HTTP status stays 200 because Praxis 0.5.3 cannot change an already-selected status during response-body processing; exact 500 conformance needs deferred-header support in Praxis core.)