[Mono] Support invoking string and array constructors on existing objects

[jkoritzinsky]

Description

The reflection tests ConstructorInfoTests.Invoke_StringConstructorWithExistingInstance and Invoke_ArrayConstructorWithExistingInstance are currently limited to CoreCLR because Mono does not support invoking string and array pseudo-constructors against existing objects.

Mono should match the expected reflection behavior for these constructor handles. The tests exercise the existing-object path and expect the runtime's documented NotSupportedException behavior to be surfaced through TargetInvocationException.

Reproduction Steps

Run these tests from src/libraries/System.Runtime/tests/System.Reflection.Tests/ConstructorInfoTests.cs on Mono:

ConstructorInfo stringConstructor = typeof(string).GetConstructor(new[] { typeof(char[]) })!;
TargetInvocationException stringException = Assert.Throws<TargetInvocationException>(() =>
    stringConstructor.Invoke("existing", new object?[] { new[] { 'n', 'e', 'w' } }));
Assert.IsType<NotSupportedException>(stringException.InnerException);

ConstructorInfo arrayConstructor = typeof(object[]).GetConstructor(new[] { typeof(int) })!;
TargetInvocationException arrayException = Assert.Throws<TargetInvocationException>(() =>
    arrayConstructor.Invoke(new object[1], new object?[] { 3 }));
Assert.IsType<NotSupportedException>(arrayException.InnerException);

Expected behavior

Both tests pass on Mono with TargetInvocationException wrapping NotSupportedException, matching CoreCLR.

Actual behavior

These constructor-on-existing-object cases are not currently supported on Mono, so the tests are guarded to CoreCLR.

Regression?

Unknown.

Known Workarounds

None.

Configuration

Mono runtime; System.Reflection.Tests.

Other information

Relevant tests: src/libraries/System.Runtime/tests/System.Reflection.Tests/ConstructorInfoTests.cs.

[!NOTE] This issue was filed with assistance from GitHub Copilot.

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @dotnet/area-system-reflection
See info in area-owners.md if you want to be subscribed.