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/claude_client.go

@@ -67,6 +67,7 @@ func NewClaudeClient(cfg *config.Config, ts *claude.ClaudeTokenStorage) *ClaudeC
 			cfg:                cfg,
 			modelQuotaExceeded: make(map[string]*time.Time),
 			tokenStorage:       ts,
+			isAvailable:        true,
 		},
 		claudeAuth:  claude.NewClaudeAuth(cfg),
 		apiKeyIndex: -1,
@@ -102,6 +103,7 @@ func NewClaudeClientWithKey(cfg *config.Config, apiKeyIndex int) *ClaudeClient {
 			cfg:                cfg,
 			modelQuotaExceeded: make(map[string]*time.Time),
 			tokenStorage:       &empty.EmptyStorage{},
+			isAvailable:        true,
 		},
 		claudeAuth:  claude.NewClaudeAuth(cfg),
 		apiKeyIndex: apiKeyIndex,
@@ -581,3 +583,13 @@ func (c *ClaudeClient) IsModelQuotaExceeded(model string) bool {
 func (c *ClaudeClient) GetRequestMutex() *sync.Mutex {
 	return nil
 }
+
+// IsAvailable returns true if the client is available for use.
+func (c *ClaudeClient) IsAvailable() bool {
+	return c.isAvailable
+}
+
+// SetUnavailable sets the client to unavailable.
+func (c *ClaudeClient) SetUnavailable() {
+	c.isAvailable = false
+}