Rename missed _projectFileCallback reference from PR1 merge

The edge case A fix in FileOperationService.AddMultiFileTab came in via
the Development merge after PR2's sed pass had already run, so it kept
the old field name. Renamed to match _loadSessionCallback.

Author: BRUNER Patrick

src/LogExpert.UI/Services/FileOperationService/FileOperationService.cs

@@ -189,8 +189,9 @@ public LogWindow AddTempFileTab (string fileName, string title)
         {
             foreach (var sessionFile in fileNames.Where(f => f.EndsWith(".lxj", StringComparison.OrdinalIgnoreCase)))
             {
-                _projectFileCallback(sessionFile, false);
+                _loadSessionCallback(sessionFile, false);
             }
+
             var logs = fileNames.Where(f => !f.EndsWith(".lxj", StringComparison.OrdinalIgnoreCase)).ToArray();
             return logs.Length > 0 ? AddMultiFileTab(logs) : null;
         }