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/openai-compatibility_client.go

@@ -68,6 +68,7 @@ func NewOpenAICompatibilityClient(cfg *config.Config, compatConfig *config.OpenA
 			httpClient:         httpClient,
 			cfg:                cfg,
 			modelQuotaExceeded: make(map[string]*time.Time),
+			isAvailable:        true,
 		},
 		compatConfig:       compatConfig,
 		currentAPIKeyIndex: apiKeyIndex,
@@ -425,3 +426,13 @@ func (c *OpenAICompatibilityClient) RefreshTokens(ctx context.Context) error {
 func (c *OpenAICompatibilityClient) GetRequestMutex() *sync.Mutex {
 	return nil
 }
+
+// IsAvailable returns true if the client is available for use.
+func (c *OpenAICompatibilityClient) IsAvailable() bool {
+	return c.isAvailable
+}
+
+// SetUnavailable sets the client to unavailable.
+func (c *OpenAICompatibilityClient) SetUnavailable() {
+	c.isAvailable = false
+}