Address Greptile review nit. DEFAULT_JUDGE_MODEL now reuses the
constant from lib/providers.py instead of duplicating the literal,
so a future identifier change only needs one edit.
The Gemini 3.1 Flash Lite preview model is being discontinued on
May 25, 2026. Per Google's GA announcement, the underlying model
architecture is identical and only the model identifier needs to
be updated from `gemini-3.1-flash-lite-preview` to
`gemini-3.1-flash-lite`.
Also relaxes the `_require_gemini_31_preview` guard to accept any
`gemini-3.1-*` identifier (renamed to `_require_gemini_31`), so the
GA name and the still-preview `gemini-3.1-pro-preview` both pass.
Jeffrey Sperling