fix(minimax): narrow image error-body except to JSON decode failures

[blackflame007]

The strict-rule gate on #31 failed with BLE001: total 2899 over limit 2898 pointing at MinimaxImageGenerationConfig._error_message_from_body, but #31 was squash-merged before the fix could land on its branch, so the blind except Exception is now on litellm_internal_staging and every subsequent PR inherits the over-budget total. This narrows the catch to the JSON decode failures the helper actually guards against, matching MinimaxVideoConfig._error_message_from_body:

    @staticmethod
    def _error_message_from_body(raw_response: httpx.Response) -> str:
        try:
            body = raw_response.json()
        except (ValueError, JSONDecodeError):
            return raw_response.text
        # ...

httpx.Response.json() raises json.JSONDecodeError (a ValueError subclass) on a non-JSON body, so the HTML-502 path this helper exists for keeps returning raw_response.text and behavior is unchanged.

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[blackflame007]

Superseded by #33, which landed the identical narrowing (JSONDecodeError import + except clause) and merged as f0343e6; the strict-rule gate on litellm_internal_staging is green again. Closing to keep the queue clean