Improve JSON parse error logging

[hemanth-asirvatham]

Motivation

• Make it easier to diagnose intermittent JSON parsing failures (e.g. the early "JSON parsing failed" bursts seen during high-concurrency runs) by surfacing the underlying parse exception in logs and error records.
• Provide richer diagnostics for JSON-mode validation so callers can see both a snippet of the invalid payload and the exact parsing error type/message.

Description

• Change parse_json_with_status in src/gabriel/utils/parsing.py to return an optional Exception alongside the parsed value and success flag via the signature -> Tuple[Optional[Union[dict, list]], bool, Optional[Exception]] so callers can access the underlying parse exception.
• Extend JSONParseError in src/gabriel/utils/openai_utils.py to accept a parse_error field and store the original exception.
• Update JSON-mode validation in get_all_responses to collect parse errors for invalid payloads and include the parse error details in the logged/emitted error text and Error Log entries so operators see Cause: <ExceptionType>: <message> plus the payload snippet.

Testing

• No automated tests were run for this change.

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Codex Task

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