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/interfaces/client.go

@@ -51,4 +51,6 @@ type Client interface {
 
 	// Provider returns the name of the AI service provider (e.g., "gemini", "claude").
 	Provider() string
+
+	RefreshTokens(ctx context.Context) error
 }