fix(moonshine): return 400 on invalid audio

[abn]

Return 400 Bad Request instead of 500 when audio format is invalid. Add integration test coverage for WAV RIFF errors and unsupported OGG.

[fl0rianr]

Thanks, this is the right direction and propagating error.status_code through set_error_response() makes sense.

I’d request one change before merge: please avoid classifying backend errors as 400 based on substrings in the error message. The backend now already returns HTTP 400 specifically for load_wav_file() failures, so the C++ wrapper can rely on the backend status code instead. With the current "Invalid"/ "Unsupported" matching, a real server-side 500 whose message happens to contain those words would be returned to clients as 400 invalid_request_error.

I’d suggest removing the message heuristic and setting invalid_request_error only for backend 4xx responses, or even only for status 400 in this PR.

Test nits:

• The OGG test currently uses OggS..., so it’s testing an invalid/corrupt file rather than a real unsupported OGG. Either rename it or use a valid OGG fixture.
• Avoid asserting the exact decoder message "Not a valid RIFF file"; the contract should be HTTP 400 + invalid_request_error.
• Consider using NamedTemporaryFile(..., suffix=...) instead of fixed temp paths.

[abn]

@fl0rianr I have committed the requested changes.

[abn]

I went ahead and updated the C++ wrapper to temporarily restore the message-based fallback heuristic for now.

The integration tests run against a precompiled binary of the Moonshine server downloaded at runtime (0.0.62, built from the main branch). Since the Python fix in tools/moonshine-server/main.py is in this PR branch and hasn't been merged or released yet, the precompiled binary used in CI still throws an uncaught exception on invalid audio uploads, resulting in an HTTP 500 response.

To keep the integration tests strict (asserting 400 Bad Request as requested) and prevent CI failures, we are temporarily mapping HTTP 500 responses containing "Not a valid RIFF file" to HTTP 400 invalid_request_error in moonshine_server.cpp.

Once this PR is merged and a new release of moonshine-server-rocm (e.g., 0.0.63) is published and pinned in backend_versions.json, we can safely remove this temporary message heuristic in a follow-up cleanup PR.

Let me know if this works for you!