chore: migrate to gemini-3.1-flash-lite GA model

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.

tests/test_evaluator_v3.py

@@ -125,7 +125,7 @@ class EvaluatorV3Tests(unittest.TestCase):
                 topic="test topic",
                 query_type="general",
                 items=[{"key": "a"}],
-                judge_model="gemini-3.1-flash-lite-preview",
+                judge_model="gemini-3.1-flash-lite",
                 gemini_api_key="key",
             )
             self.assertEqual({"a": 3}, cached)
@@ -136,7 +136,7 @@ class EvaluatorV3Tests(unittest.TestCase):
                 topic="test topic",
                 query_type="general",
                 items=[],
-                judge_model="gemini-3.1-flash-lite-preview",
+                judge_model="gemini-3.1-flash-lite",
                 gemini_api_key=None,
             )
             self.assertEqual({}, skipped)