Guard nvcf 500 error parsing against non-JSON bodies

[chuenchen309]

On an HTTP 500, NvcfChat._call_model parsed the error body unconditionally:

if response.status_code == 500 and json.loads(response.content)["detail"].startswith("Input value error"):

A 500 body is not guaranteed to be JSON — a gateway/proxy/load balancer in front of the NVCF function commonly returns an HTML or plain-text error page — and even when JSON it may not carry a string detail. In those cases this line raises an uncaught json.JSONDecodeError (a ValueError), KeyError, or AttributeError.

None of those is in the @backoff.on_exception tuple (AttributeError is, but JSONDecodeError/KeyError are not, and the string .startswith on a non-str detail is the AttributeError path), so on a long scan a single such 500 propagates out of _call_model and aborts the whole run instead of being retried or skipped — the top-level handler just logs and exits.

Fix

Extract the check into a defensive _is_input_value_error(response) helper. Anything that can't be confirmed as an input-value error (non-JSON body, missing/non-string detail, or a different message) returns False and falls through to the existing raise_for_status() path — i.e. it's treated as a genuine server error and retried by backoff, exactly as any other unrecognized 5xx already is. Behaviour for a well-formed "Input value error" 500 (skip → [None]) is unchanged.

Verification

• Added test_nvcf_is_input_value_error_parses_defensively, parametrized over: a well-formed input-value error (→ True), a different message, a non-JSON body, an empty body, JSON without detail, and a non-string detail (all → False). Each of the last four would previously have raised inside the parse. Full tests/generators/test_nvcf.py passes; black --check clean.
• garak -t <target_type> -n <model_name> — not run: this is a unit-level fix and I have no live target configured; verified via the tests above rather than ticking a box I didn't exercise.
• Supporting configuration such as a generator configuration file — n/a, no config surface changed.

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Disclosure: this contribution was prepared with the assistance of an AI agent (Claude Code). The analysis, fix, and tests were reviewed by me before submission.