improve: enhance VM disk usage detection for Windows guests (addresses #416)

- Add debug logging to guest agent filesystem API responses
- Better handle Windows drive mountpoints (C:\, D:\, etc.)
- Improve empty filesystem list detection and logging
- Add specific handling for Windows filesystems that may report differently

This should help diagnose why some VMs with guest agents installed still show 0% or missing disk usage, particularly on Windows systems.

internal/monitoring/monitor_optimized.go

@@ -292,7 +292,10 @@ func (m *Monitor) pollVMsWithNodesOptimized(ctx context.Context, instanceName st
 										Msg("Processing filesystem from guest agent")
 									
 									// Skip special filesystems and Windows System Reserved
-									if fs.Type == "tmpfs" || fs.Type == "devtmpfs" || 
+									// For Windows, mountpoints are like "C:\\" or "D:\\" - don't skip those
+									isWindowsDrive := len(fs.Mountpoint) >= 2 && fs.Mountpoint[1] == ':' && strings.Contains(fs.Mountpoint, "\\")
+									
+									if !isWindowsDrive && (fs.Type == "tmpfs" || fs.Type == "devtmpfs" || 
 									   strings.HasPrefix(fs.Mountpoint, "/dev") ||
 									   strings.HasPrefix(fs.Mountpoint, "/proc") ||
 									   strings.HasPrefix(fs.Mountpoint, "/sys") ||
@@ -300,7 +303,7 @@ func (m *Monitor) pollVMsWithNodesOptimized(ctx context.Context, instanceName st
 									   fs.Mountpoint == "/boot/efi" ||
 									   fs.Mountpoint == "System Reserved" ||
 									   strings.Contains(fs.Mountpoint, "System Reserved") ||
-									   strings.HasPrefix(fs.Mountpoint, "/snap") { // Skip snap mounts
+									   strings.HasPrefix(fs.Mountpoint, "/snap")) { // Skip snap mounts
 										log.Debug().
 											Str("vm", vm.Name).
 											Str("mountpoint", fs.Mountpoint).

pkg/proxmox/client.go

@@ -913,6 +913,13 @@ func (c *Client) GetVMFSInfo(ctx context.Context, node string, vmid int) ([]VMFi
 		return nil, err
 	}
 	
+	// Log the raw response for debugging
+	log.Debug().
+		Str("node", node).
+		Int("vmid", vmid).
+		Str("response", string(bodyBytes)).
+		Msg("Raw response from guest agent get-fsinfo")
+	
 	// Try to unmarshal as an array first (expected format)
 	var arrayResult struct {
 		Data struct {
@@ -961,10 +968,20 @@ func (c *Client) GetVMFSInfo(ctx context.Context, node string, vmid int) ([]VMFi
 	}
 	if err := json.Unmarshal(bodyBytes, &objectResult); err == nil {
 		// If result is an object, it might be an error or empty response
+		// Check if it's null or an error
+		if objectResult.Data.Result == nil {
+			log.Debug().
+				Str("node", node).
+				Int("vmid", vmid).
+				Msg("GetVMFSInfo received null result - guest agent may not be providing disk info")
+		} else {
+			log.Debug().
+				Str("node", node).
+				Int("vmid", vmid).
+				Interface("result", objectResult.Data.Result).
+				Msg("GetVMFSInfo received object instead of array")
+		}
 		// Return empty array to indicate no filesystem info available
-		log.Debug().
-			Interface("result", objectResult.Data.Result).
-			Msg("GetVMFSInfo received object instead of array, returning empty")
 		return []VMFileSystem{}, nil
 	}