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/client/qwen_client.go

@@ -61,6 +61,7 @@ func NewQwenClient(cfg *config.Config, ts *qwen.QwenTokenStorage) *QwenClient {
 			cfg:                cfg,
 			modelQuotaExceeded: make(map[string]*time.Time),
 			tokenStorage:       ts,
+			isAvailable:        true,
 		},
 		qwenAuth: qwen.NewQwenAuth(cfg),
 	}
@@ -447,3 +448,13 @@ func (c *QwenClient) IsModelQuotaExceeded(model string) bool {
 func (c *QwenClient) GetRequestMutex() *sync.Mutex {
 	return nil
 }
+
+// IsAvailable returns true if the client is available for use.
+func (c *QwenClient) IsAvailable() bool {
+	return c.isAvailable
+}
+
+// SetUnavailable sets the client to unavailable.
+func (c *QwenClient) SetUnavailable() {
+	c.isAvailable = false
+}