Refactor collection initializers to use C# 12 collection expressions

This commit updates various files across the Core and WebApp projects to replace traditional collection initializers with C# 12 collection expressions. Changes include modifications to EventAssignment.cs, TeamScheduler_DecisionTree.cs, CareerField.cs, EventDefinition.cs, and several components in the WebApp. These updates enhance code readability and maintainability by adhering to modern C# syntax standards.

WebApp/Components/Features/Events/CareerMapping.razor

@@ -118,7 +118,7 @@ else
                 {
                     if (!_fieldIdToCareers.ContainsKey(field.Id))
                     {
-                        _fieldIdToCareers[field.Id] = new List<string>();
+                        _fieldIdToCareers[field.Id] = [];
                     }
                     // Add unique career names for this field
                     foreach (var career in evt.RelatedCareers)
@@ -153,8 +153,8 @@ else
 
     private NetworkData GenerateNetworkData(List<EventDefinition> events)
     {
-        var nodes = new List<Node>();
-        var edges = new List<Edge>();
+        List<Node> nodes = [];
+        List<Edge> edges = [];
 
         // Dictionary to track node IDs (to avoid duplicates)
         var eventNodeIds = new Dictionary<int, string>();
@@ -284,7 +284,7 @@ else
                         Title = field.Name,
                         Description = field.Description,
                         IsCareerField = true,
-                        Careers = careers?.ToList() ?? new List<string>()
+                        Careers = careers?.ToList() ?? []
                     };
                 }
             }