Update Gemini fallback models to latest 2.5 series (flash, pro, flash-lite)

Co-authored-by: LeafCreeper <120477804+LeafCreeper@users.noreply.github.com>

Author: Copilot

src/config.py

@@ -34,9 +34,9 @@
 GEMINI_API_KEY = os.environ.get("GEMINI_API_KEY", "")
 GEMINI_BASE_URL = "https://generativelanguage.googleapis.com/v1beta/openai/"
 GEMINI_MODELS = [
-    "gemini-2.0-flash",
-    "gemini-2.5-flash-preview-05-20",
-    "gemini-1.5-pro",
+    "gemini-2.5-flash",
+    "gemini-2.5-pro",
+    "gemini-2.5-flash-lite",
 ]
 
 # QQ SMTP