fix: 修复claude OAuth账户刷新token失败的bug

backend/internal/repository/claude_oauth_service.go

@@ -199,16 +199,20 @@ func (s *claudeOAuthService) ExchangeCodeForToken(ctx context.Context, code, cod
 func (s *claudeOAuthService) RefreshToken(ctx context.Context, refreshToken, proxyURL string) (*oauth.TokenResponse, error) {
 	client := s.clientFactory(proxyURL)
 
-	formData := url.Values{}
-	formData.Set("grant_type", "refresh_token")
-	formData.Set("refresh_token", refreshToken)
-	formData.Set("client_id", oauth.ClientID)
+	// 使用 JSON 格式（与 ExchangeCodeForToken 保持一致）
+	// Anthropic OAuth API 期望 JSON 格式的请求体
+	reqBody := map[string]any{
+		"grant_type":    "refresh_token",
+		"refresh_token": refreshToken,
+		"client_id":     oauth.ClientID,
+	}
 
 	var tokenResp oauth.TokenResponse
 
 	resp, err := client.R().
 		SetContext(ctx).
-		SetFormDataFromValues(formData).
+		SetHeader("Content-Type", "application/json").
+		SetBody(reqBody).
 		SetSuccessResult(&tokenResp).
 		Post(s.tokenURL)