feat: Add new experimental Gemini versions to ModelList

- Included additional versions: "gemini-2.0-flash-thinking-exp" and "gemini-2.0-flash-thinking-exp-1219".
- Added comments to categorize versions as old, experimental, and flash experimental for better clarity.

relay/channel/gemini/constant.go

@@ -6,9 +6,14 @@ const (
 
 var ModelList = []string{
 	"gemini-1.5-pro-latest", "gemini-1.5-flash-latest",
+	// old version
 	"gemini-1.5-pro-exp-0827", "gemini-1.5-flash-exp-0827",
+	// exp
 	"gemini-exp-1114", "gemini-exp-1121", "gemini-exp-1206",
+	// flash exp
 	"gemini-2.0-flash-exp",
+	// thinking exp
+	"gemini-2.0-flash-thinking-exp",
 	"gemini-2.0-flash-thinking-exp-1219",
 }