Disable JSON polymorphism source-gen tests on NativeAOT

The 12 test methods (6 [Fact] and 6 [Theory]) in
PolymorphicTests.CustomTypeHierarchies.cs that use
CustomPolymorphismResolver rely on runtime reflection-based type
resolution which is not supported on NativeAOT. Replace [Fact] with
[ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
and [Theory] with [ConditionalTheory(typeof(PlatformDetection),
nameof(PlatformDetection.IsNotNativeAot))] on those 12 methods.

The skip applies to all 4 SourceGeneration subclasses
(PolymorphicTests_Metadata, PolymorphicTests_Metadata_AsyncStream,
PolymorphicTests_Default, PolymorphicTests_Default_AsyncStream) since
they all inherit from the same abstract base class.

Tests verified: 549 total pass on CoreCLR (including the 12 affected
methods' 56 theory cases), 0 failures.

Co-authored-by: MichalStrehovsky <13110571+MichalStrehovsky@users.noreply.github.com>

Author: Copilot

src/libraries/System.Text.Json/tests/Common/PolymorphicTests.CustomTypeHierarchies.cs

@@ -129,7 +129,7 @@ public async Task PolymorphicClass_AllowOutOfOrderMetadata_RejectsInvalidInputs(
 
         //--
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         [MemberData(nameof(Get_PolymorphicClass_CustomConfigWithBaseTypeFallback_TestData_Serialization))]
         public Task PolymorphicClass_CustomConfigWithBaseTypeFallback_TestData_Serialization(PolymorphicClass.TestData testData)
             => TestMultiContextSerialization(
@@ -142,7 +142,7 @@ public Task PolymorphicClass_CustomConfigWithBaseTypeFallback_TestData_Serializa
         public static IEnumerable<object[]> Get_PolymorphicClass_CustomConfigWithBaseTypeFallback_TestData_Serialization()
             => PolymorphicClass.GetSerializeTestData_CustomConfigWithBaseTypeFallback().Select(entry => new object[] { entry });
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         [MemberData(nameof(Get_PolymorphicClass_CustomConfigWithBaseTypeFallback_TestData_Deserialization))]
         public Task PolymorphicClass_CustomConfigWithBaseTypeFallback_TestData_Deserialization(PolymorphicClass.TestData testData)
             => TestMultiContextDeserialization<PolymorphicClass>(
@@ -157,7 +157,7 @@ public static IEnumerable<object[]> Get_PolymorphicClass_CustomConfigWithBaseTyp
                 .Where(entry => entry.ExpectedJson != null)
                 .Select(entry => new object[] { entry });
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         public async Task PolymorphicClass_CustomConfigWithBaseTypeFallback_TestDataArray_Serialization()
         {
             IEnumerable<(PolymorphicClass Value, string ExpectedJson)> inputs =
@@ -168,7 +168,7 @@ public async Task PolymorphicClass_CustomConfigWithBaseTypeFallback_TestDataArra
             await TestMultiContextSerialization(inputs, options: PolymorphicClass.CustomConfigWithBaseTypeFallback);
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         public async Task PolymorphicClass_CustomConfigWithBaseTypeFallbacks_TestDataArray_Deserialization()
         {
             IEnumerable<(string ExpectedJson, PolymorphicClass ExpectedRoundtripValue)> inputs =
@@ -182,7 +182,7 @@ await TestMultiContextDeserialization(
                 options: PolymorphicClass.CustomConfigWithBaseTypeFallback);
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         [InlineData("$['$type']", """{ "$type" : "derivedClass1", "Number" : 42 }""")]
         [InlineData("$._case", """{ "_case" : "derivedClass1", "_case" : "derivedClass1", "Number" : 42 }""")]
         [InlineData("$._case", """{ "_case" : "derivedClass1", "Number" : 42, "_case" : "derivedClass1"}""")]
@@ -206,7 +206,7 @@ public async Task PolymorphicClass_CustomConfigWithBaseTypeFallback_InvalidTypeD
 
         //---
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         [MemberData(nameof(Get_PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestData_Serialization))]
         public Task PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestData_Serialization(PolymorphicClass.TestData testData)
             => TestMultiContextSerialization(
@@ -219,7 +219,7 @@ public Task PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestData_Se
         public static IEnumerable<object[]> Get_PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestData_Serialization()
             => PolymorphicClass.GetSerializeTestData_CustomConfigWithNearestAncestorFallback().Select(entry => new object[] { entry });
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         [MemberData(nameof(Get_PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestData_Deserialization))]
         public Task PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestData_Deserialization(PolymorphicClass.TestData testData)
             => TestMultiContextDeserialization<PolymorphicClass>(
@@ -234,7 +234,7 @@ public static IEnumerable<object[]> Get_PolymorphicClass_CustomConfigWithNearest
                     .Where(entry => entry.ExpectedJson != null)
                     .Select(entry => new object[] { entry });
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         public async Task PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestDataArray_Serialization()
         {
             IEnumerable<(PolymorphicClass Value, string ExpectedJson)> inputs =
@@ -245,7 +245,7 @@ public async Task PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestD
             await TestMultiContextSerialization(inputs, options: PolymorphicClass.CustomConfigWithNearestAncestorFallback);
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         public async Task PolymorphicClass_CustomConfigWithNearestAncestorFallback_TestDataArray_Deserialization()
         {
             IEnumerable<(string ExpectedJson, PolymorphicClass ExpectedRoundtripValue)> inputs =
@@ -259,7 +259,7 @@ await TestMultiContextDeserialization(
                 options: PolymorphicClass.CustomConfigWithNearestAncestorFallback);
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         [InlineData("$['$type']", """{ "$type" : "derivedClass1", "Number" : 42 }""")]
         [InlineData("$._case", """{ "_case" : "derivedClass1", "_case" : "derivedClass1", "Number" : 42 }""")]
         [InlineData("$._case", """{ "_case" : "derivedClass1", "Number" : 42, "_case" : "derivedClass1"}""")]
@@ -1944,7 +1944,7 @@ public static IEnumerable<TestData> GetSerializeTestData()
             public record TestData(PolymorphicDictionary Value, string ExpectedJson, PolymorphicDictionary ExpectedRoundtripValue);
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         public async Task PolymorphicDictionaryInterface_Serialization()
         {
             var values = new IEnumerable<KeyValuePair<int, object>>[]
@@ -1968,7 +1968,7 @@ public async Task PolymorphicDictionaryInterface_Serialization()
             JsonTestHelper.AssertJsonEqual(expectedJson, actualJson);
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNativeAot))]
         public async Task PolymorphicDictionaryInterface_Deserialization()
         {
             string json =