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/Calendar/Index.razor

@@ -99,7 +99,7 @@
             // Load teams for all event definitions
             var teamsByEventId = await EventOccurrenceService.GetTeamsByEventDefinitionIdsAsync(eventDefinitionIds);
             
-            var items = new List<CalendarEventItem>();
+            List<CalendarEventItem> items = [];
             foreach (var occ in eventOccurrences)
             {
                 try
@@ -112,7 +112,7 @@
                     // Get student first names for this event definition
                     var studentFirstNames = occ.EventDefinition != null 
                         ? StudentFirstNames(occ.EventDefinition, teamsByEventId)
-                        : new List<string>();
+                        : [];
 
                     var calendarItem = new CalendarEventItem(occ, studentFirstNames);
                     items.Add(calendarItem);
@@ -135,7 +135,7 @@
         catch (Exception ex)
         {
             Logger.LogError(ex, "Error loading calendar events");
-            _calendarItems = new List<CalendarEventItem>();
+            _calendarItems = [];
         }
         finally
         {
@@ -145,7 +145,7 @@
 
     private static List<string> StudentFirstNames(EventDefinition ed, Dictionary<int, List<Team>> teamsByEventId)
     {
-        var studentFirstNames = new List<string>();
+        List<string> studentFirstNames = [];
         if (ed?.Id == null || !teamsByEventId.TryGetValue(ed.Id, out var teams)) return studentFirstNames;
 
         // Get all unique student first names from all teams for this event
@@ -220,7 +220,7 @@
 
     private string GetEventTooltip(CalendarEventItem item)
     {
-        var parts = new List<string>();
+        List<string> parts = [];
         
         if (!string.IsNullOrEmpty(item.EventDefinition?.Name))
         {