Add GeminiGetHandler, enhance Gemini functionality, and enable token counting

- Added `GeminiGetHandler` for handling GET requests with extended Gemini model support.
- Introduced `geminiCountTokens` function to calculate token usage.
- Refactored `APIRequest` and related methods to support `alt` parameter for enhanced flexibility.
- Updated routes and request processing to integrate new handler and functions.

internal/api/server.go

@@ -77,6 +77,7 @@ func (s *Server) setupRoutes() {
 	{
 		v1beta.GET("/models", s.handlers.GeminiModels)
 		v1beta.POST("/models/:action", s.handlers.GeminiHandler)
+		v1beta.GET("/models/:action", s.handlers.GeminiGetHandler)
 	}
 
 	// Root endpoint