Add token refresh handling for 401 responses across clients

- Implemented `RefreshTokens` method in client interfaces and Gemini clients.
- Updated handlers to call `RefreshTokens` on 401 responses and retry requests if token refresh succeeds.
- Enhanced error handling and retry logic to accommodate token refresh flow.

internal/client/gemini_client.go

@@ -434,3 +434,8 @@ func (c *GeminiClient) GetUserAgent() string {
 func (c *GeminiClient) GetRequestMutex() *sync.Mutex {
 	return nil
 }
+
+func (c *GeminiClient) RefreshTokens(ctx context.Context) error {
+	// API keys don't need refreshing
+	return nil
+}