Use JSON source generator in WebApplicationFactory (#55595)

Use JSON source generator to deserialize dictionary.
Resolves #55586.

src/Mvc/Mvc.Testing/src/WebApplicationFactory.cs

@@ -6,6 +6,7 @@
 using System.Net.Http;
 using System.Reflection;
 using System.Text.Json;
+using System.Text.Json.Serialization;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Hosting.Server;
 using Microsoft.AspNetCore.TestHost;
@@ -21,7 +22,7 @@ namespace Microsoft.AspNetCore.Mvc.Testing;
 /// </summary>
 /// <typeparam name="TEntryPoint">A type in the entry point assembly of the application.
 /// Typically the Startup or Program classes can be used.</typeparam>
-public class WebApplicationFactory<TEntryPoint> : IDisposable, IAsyncDisposable where TEntryPoint : class
+public partial class WebApplicationFactory<TEntryPoint> : IDisposable, IAsyncDisposable where TEntryPoint : class
 {
     private bool _disposed;
     private bool _disposedAsync;
@@ -235,7 +236,7 @@ private void SetContentRoot(IWebHostBuilder builder)
 
     private static string? GetContentRootFromFile(string file)
     {
-        var data = JsonSerializer.Deserialize<IDictionary<string, string>>(File.ReadAllBytes(file))!;
+        var data = JsonSerializer.Deserialize(File.ReadAllBytes(file), CustomJsonSerializerContext.Default.IDictionaryStringString)!;
         var key = typeof(TEntryPoint).Assembly.GetName().FullName;
 
         // If the `ContentRoot` is not provided in the app manifest, then return null
@@ -248,6 +249,9 @@ private void SetContentRoot(IWebHostBuilder builder)
         return (contentRoot == "~") ? AppContext.BaseDirectory : contentRoot;
     }
 
+    [JsonSerializable(typeof(IDictionary<string, string>))]
+    private sealed partial class CustomJsonSerializerContext : JsonSerializerContext;
+
     private string? GetContentRootFromAssembly()
     {
         var metadataAttributes = GetContentRootMetadataAttributes(