fix(translation): fail-closed on malformed Chat Completions success

[leseb]

Summary

The Responses translator accepted any HTTP 200 body as a successful Chat Completions response, so a malformed payload like {} became a counterfeit completed Responses object (status: "completed", error: null, output: []) that silently hid the upstream failure. This change validates the minimum successful shape before translating — a non-empty choices array, a first choice object, a supported finish_reason (stop/length/tool_calls/content_filter), an assistant message, and at least one translatable output (content, refusal, or a well-formed function call, with tool_calls finish reasons requiring function calls) — so malformed responses now fail closed as HTTP 500 instead of a fake success. It is the smallest complete change: the check lives in the shared translator that every finite HTTP 200 path already calls, and well-formed responses are unaffected.

Related issue

Closes #804

Validation

• Unit tests — cargo test -p praxis-ai-apis -- translation responses_to_chat_completions (138 passed; 5 new/updated shape-rejection tests confirmed by exact name)
• Integration or functional tests — cargo test -p praxis-tests-integration -- responses_to_chat_completions (7 passed, incl. new responses_to_chat_completions_rejects_malformed_finite_success asserting a finite HTTP 200 {} → 500)
• make lint (clippy -D warnings, fmt, deps, docs, example, and README-sync checks all green)
• make build

Checklist

• I reviewed every changed line and can explain the change.
• New capabilities include an example config and functional example test. (No new capability — a bug fix; added a functional example test to the existing responses_to_chat_completions example config.)
• User-facing behavior and generated documentation are updated. (Malformed finite success now returns 500; generated docs remain in sync per make lint README-sync checks — no doc changes needed.)
• Performance-sensitive changes include appropriate benchmark or load-test evidence. (N/A — validation is a bounded check over an already-parsed value.)
• Commits are signed and include a Signed-off-by trailer.

Breaking changes

Narrow behavioral change on an error path: a backend returning HTTP 200 with a body that does not conform to the Chat Completions success schema previously produced a completed Responses object with empty output; it now fails closed with HTTP 500. Well-formed responses are unaffected and no migration is required.