Fix net481 build break and private-constructor assert regressions from PR #130503

[Copilot]

PR #130503 gated STJ member accessor selection on RuntimeFeature.IsDynamicCodeCompiled and routed interpreter-only runtimes (WASM/iOS/Mono interpreter) through ReflectionMemberAccessor. This broke main in two ways, reported in comment #4957325583 and comment #4957354177.

Description

• net481 compile break (CS0117) — ExceptionTests.cs referenced RuntimeFeature.IsDynamicCodeCompiled, which does not exist on .NET Framework. [SkipOnTargetFramework(NetFramework)] only skips at runtime, so the file still failed to compile. Guarded the reflection-accessor test with #if NET (the scenario is meaningless on .NET Framework, where dynamic code is always compiled).

• Private-constructor assert crash on the reflection/emit accessors — routing to ReflectionMemberAccessor surfaced a latent Debug.Assert(... constructor.IsPublic ...) that aborts on supported private [JsonConstructor] constructors (e.g. PrivateParameterizedCtor_WithAttribute). Removed the invalid IsPublic clause from the asserts in ReflectionMemberAccessor (3 sites) and ReflectionEmitMemberAccessor (2 sites — it already invokes private ctors via skipVisibility: true), keeping the DeclaringType/!IsStatic checks.

// A private [JsonConstructor] is a supported scenario the assert wrongly rejected:
class PrivateParameterizedCtor_WithAttribute
{
    public int X { get; }
    [JsonConstructor]
    private PrivateParameterizedCtor_WithAttribute(int x) => X = x;
}

[Copilot]

Pull request overview

Fixes regressions in System.Text.Json introduced by accessor selection changes: restores net481 test compilation and removes invalid debug assertions that rejected supported private [JsonConstructor] scenarios when using reflection-based accessors.

Changes:

• Guard the reflection-accessor RemoteExecutor regression test behind #if NET to avoid RuntimeFeature.IsDynamicCodeCompiled references on .NET Framework.
• Relax Debug.Assert constructor visibility checks in ReflectionMemberAccessor and ReflectionEmitMemberAccessor so private (but supported) constructors don’t trigger debug-time aborts.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/ExceptionTests.cs	Wraps the reflection-accessor test in #if NET so net481 builds compile successfully.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/ReflectionMemberAccessor.cs	Removes constructor.IsPublic from constructor assertions to allow private [JsonConstructor] delegates without debug asserts firing.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/ReflectionEmitMemberAccessor.cs	Removes constructor.IsPublic from constructor assertions; skipVisibility: true already supports invoking non-public ctors.

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