chore: update OpenRouter and provider model lists

OpenRouter / _FALLBACK_MODELS (8 → 7 models):
- Remove o4-mini (reasoning specialist, not a general-purpose pick)
- Remove claude-sonnet-4-5 (superseded by 4.6)
- Add gemini-3-flash as fast/cheap Google option
- Update gemini-2.5-pro → gemini-3.1-pro (current flagship)
- Better provider labels (Google, DeepSeek, Meta instead of 'Other')

Direct-API providers:
- openai: replace o4-mini with gpt-5.4 (general-purpose pairing with Mini)
- google / gemini: gemini-2.5-pro → 3.1-pro, gemini-2.0-flash → 3-flash
- Copilot, Nous, opencode-zen: same Gemini updates throughout

Test: update test_fallback_still_has_o4_mini → test_fallback_has_gpt54

tests/test_issues_373_374_375.py

@@ -127,10 +127,12 @@ class TestStaleModelListCleanup:
             "_FALLBACK_MODELS must keep gpt-5.4-mini as primary OpenAI model (#374)"
         )
 
-    def test_fallback_still_has_o4_mini(self):
-        """_FALLBACK_MODELS must still contain o4-mini (reasoning model)."""
-        assert "o4-mini" in CONFIG_PY, (
-            "_FALLBACK_MODELS must keep o4-mini as reasoning model (#374)"
+    def test_fallback_has_gpt54(self):
+        """_FALLBACK_MODELS must contain gpt-5.4-mini as the primary OpenAI option."""
+        from api.config import _FALLBACK_MODELS
+        ids = [m["id"] for m in _FALLBACK_MODELS]
+        assert any("gpt-5.4-mini" in mid for mid in ids), (
+            "_FALLBACK_MODELS must include gpt-5.4-mini as the primary OpenAI option"
         )
 
     def test_copilot_list_unchanged(self):