Clarify defaultRefAssemblies list capacity in AddType.cs (#12520)

# PR Summary

* Clarify calculation of `defaultRefAssemblies` initial list capacity.
* Assert if list capacity is increased as a result of a resize.

## PR Context

InitDefaultRefAssemblies may initialize a list too small to hold the reference assemblies distributed with netcoreapp5.0, which may result in an expensive reallocation of the internal array.

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src/Microsoft.PowerShell.Commands.Utility/commands/utility/AddType.cs

@@ -668,14 +668,26 @@ namespace Microsoft.PowerShell.Commands
         /// </summary>
         private static IEnumerable<PortableExecutableReference> InitDefaultRefAssemblies()
         {
-            // netcoreapp3.0 currently comes with 148 reference assemblies (maybe more in future), so we use a capacity of '150'.
-            var defaultRefAssemblies = new List<PortableExecutableReference>(150);
+            // Define number of reference assemblies distributed with PowerShell.
+            // This number is accurate as of PowerShell v7.1.0-preview.1 built with .NET v5.0.100-preview.1.20155.7
+            const int numberOfPowershellRefAssemblies = 151;
+
+            const int capacity = numberOfPowershellRefAssemblies + 1;
+            var defaultRefAssemblies = new List<PortableExecutableReference>(capacity);
 
             foreach (string file in Directory.EnumerateFiles(s_netcoreAppRefFolder, "*.dll", SearchOption.TopDirectoryOnly))
             {
                 defaultRefAssemblies.Add(MetadataReference.CreateFromFile(file));
             }
+
+            // Add System.Management.Automation.dll
             defaultRefAssemblies.Add(MetadataReference.CreateFromFile(typeof(PSObject).Assembly.Location));
+
+            // We want to avoid reallocating the internal array, so we assert if the list capacity has increased.
+            Diagnostics.Assert(
+                defaultRefAssemblies.Capacity <= capacity,
+                $"defaultRefAssemblies was resized because of insufficient initial capacity! A capacity of {defaultRefAssemblies.Count} is required.");
+
             return defaultRefAssemblies;
         }