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/gemini-web_client.go

@@ -96,6 +96,7 @@ func NewGeminiWebClient(cfg *config.Config, ts *gemini.GeminiWebTokenStorage, to
 			cfg:                cfg,
 			tokenStorage:       ts,
 			modelQuotaExceeded: make(map[string]*time.Time),
+			isAvailable:        true,
 		},
 		tokenFilePath:  tokenFilePath,
 		convStore:      make(map[string][]string),
@@ -1072,3 +1073,13 @@ func (c *GeminiWebClient) storeConversationJSON(model string, history []geminiWe
 	c.convMutex.Unlock()
 	_ = geminiWeb.SaveConvData(geminiWeb.ConvDataPath(c.tokenFilePath), items, index)
 }
+
+// IsAvailable returns true if the client is available for use.
+func (c *GeminiWebClient) IsAvailable() bool {
+	return c.isAvailable
+}
+
+// SetUnavailable sets the client to unavailable.
+func (c *GeminiWebClient) SetUnavailable() {
+	c.isAvailable = false
+}