made Error fully immutable

Raliway.Tests/Errors/Serialization.cs

@@ -1,3 +1,5 @@
+using System.Collections.Immutable;
+
 namespace Railway.Tests.Errors;
 
 public class Serialization
@@ -8,9 +10,8 @@ public class Serialization
         // Given
         Error many_errors = new ManyErrors(new Error[]{
             new ExpectedError("err1", "msg1"){
-                ExtensionData = {
-                    ["ext"] = "ext_value"
-                }
+                ExtensionData = ImmutableDictionary<string, string>.Empty
+                    .Add("ext", "ext_value"),
             },
             new ExceptionalError(new Exception("msg2")),
         });
@@ -18,7 +19,7 @@ public class Serialization
         var result = JsonSerializer.Serialize(many_errors);
         // Then
         Assert.Equal(
-            expected: "[{\"$$err\":0,\"Type\":\"err1\",\"Message\":\"msg1\",\"ext\":\"ext_value\"},{\"$$err\":1,\"Type\":\"Exception\",\"Message\":\"msg2\"}]",
+            expected: "[{\"$$err\":0,\"Type\":\"err1\",\"Message\":\"msg1\",\"ExtensionData\":{\"ext\":\"ext_value\"}},{\"$$err\":1,\"Type\":\"Exception\",\"Message\":\"msg2\"}]",
             result);
     }
 
@@ -26,7 +27,7 @@ public class Serialization
     public void WhenDeserializingManyErrors()
     {
         // Given
-        var json = "[{\"$$err\":0,\"Type\":\"err1\",\"Message\":\"msg1\",\"ext\":\"ext_value\"},{\"$$err\":1,\"Type\":\"Exception\",\"Message\":\"msg2\"}]";
+        var json = "[{\"$$err\":0,\"Type\":\"err1\",\"Message\":\"msg1\",\"ExtensionData\":{\"ext1\":\"ext_value1\",\"ext2\":\"ext_value2\"}},{\"$$err\":1,\"Type\":\"Exception\",\"Message\":\"msg2\"}]";
         // When
         var result = JsonSerializer.Deserialize<Error[]>(json);
         // Then
@@ -39,7 +40,10 @@ public class Serialization
             result
         );
         Assert.Equal(
-            expected: "ext_value",
-            result[0].ExtensionData["ext"].ToString());
+            expected: "ext_value1",
+            result[0]["ext1"]);
+        Assert.Equal(
+            expected: "ext_value2",
+            result[0]["ext2"]);
     }
 }