feat: add client availability tracking and error handling improvements

- Introduced `IsAvailable` and `SetUnavailable` methods to clients for availability tracking.
- Integrated availability checks in client selection logic to skip unavailable clients.
- Enhanced error handling by marking clients unavailable on specific error codes (e.g., 401, 402).
- Removed redundant quota verification logs in client reordering logic.

internal/interfaces/client.go

@@ -52,5 +52,12 @@ type Client interface {
 	// Provider returns the name of the AI service provider (e.g., "gemini", "claude").
 	Provider() string
 
+	// RefreshTokens refreshes the access tokens if needed
 	RefreshTokens(ctx context.Context) error
+
+	// IsAvailable returns true if the client is available for use.
+	IsAvailable() bool
+
+	// SetUnavailable sets the client to unavailable.
+	SetUnavailable()
 }