Refactor executor error handling and usage reporting

- Updated the Execute methods in various executors (GeminiCLIExecutor, GeminiExecutor, IFlowExecutor, OpenAICompatExecutor, QwenExecutor) to return a response and error as named return values for improved clarity.
- Enhanced error handling by deferring failure tracking in usage reporters, ensuring that failures are reported correctly.
- Improved response body handling by ensuring proper closure and error logging for HTTP responses across all executors.
- Added failure tracking and reporting in the usage reporter to capture unsuccessful requests.
- Updated the usage logging structure to include a 'Failed' field for better tracking of request outcomes.
- Adjusted the logic in the RequestStatistics and Record methods to accommodate the new failure tracking mechanism.

internal/api/handlers/management/usage.go

@@ -13,5 +13,8 @@ func (h *Handler) GetUsageStatistics(c *gin.Context) {
 	if h != nil && h.usageStats != nil {
 		snapshot = h.usageStats.Snapshot()
 	}
-	c.JSON(http.StatusOK, gin.H{"usage": snapshot})
+	c.JSON(http.StatusOK, gin.H{
+		"usage":           snapshot,
+		"failed_requests": snapshot.FailureCount,
+	})
 }