Modernises the UTF8 string marshalling interop tests by splitting the single monolithic [Fact] into parameterised [Theory] tests, replacing native throw with BOOL return values, and consolidating all test classes into StringMarshaling.UTF8Tests.

Description

C# (UTF8Test.cs)

• Merged UTF8StringTests, UTF8StringBuilderTests, UTF8StructMarshalling, UTF8DelegateMarshalling, and Test into a single StringMarshaling.UTF8Tests class
• Replaced the single TestEntryPoint loop-driven [Fact] with individual [Theory] methods backed by Utf8StringsWithIndex() / NonNullUtf8StringsWithIndex() static [MemberData] sources
• Updated DllImport declarations for StringParameterRef, StringBuilderParameterInOut, and TestStructWithUtf8Field to return bool with [return: MarshalAs(UnmanagedType.Bool)]
• All assertions use Assert.* instead of throw new Exception
• Split TestUTF8StructMarshalling into TestStructWithUtf8FieldParameter ([Theory]), TestSetStringInStruct ([Fact]), and CompareWithUTF8Encoding ([Fact], non-Windows only)
• Fixed TestOutStringParameter to call StringParameterOut (was incorrectly calling StringParameterInOut)
• TestEmptyString now asserts the returned string is null (empty input → native returns null pointer)

Native (UTF8TestNative.cpp)

• StringBuilderParameterInOut, TestStructWithUtf8Field, StringParameterRef: void → BOOL; validation failures return FALSE instead of throw
• utf16_to_utf8 / utf8_to_utf16 helpers: throw on conversion failure → return nullptr; allocated buffers are now freed before returning nullptr on the error path
• Fixed memory leaks on all FALSE/nullptr error paths: utf16_to_utf8, utf8_to_utf16, StringBuilderParameterInOut, TestStructWithUtf8Field, and StringParameterRef all free their allocated buffers before returning on failure

Before / After

// Before: one Fact running everything in loops
[Fact]
public static void TestEntryPoint() {
    for (int i = 0; i < utf8Strings.Length; i++)
        UTF8StringTests.TestInOutStringParameter(utf8Strings[i], i);
    // ...
}

// After: individual Theory per scenario
[Theory]
[MemberData(nameof(Utf8StringsWithIndex))]
public static void TestInOutStringParameter(string orgString, int index)
{
    string nativeString = StringParameterInOut(orgString, index);
    Assert.Equal(orgString, nativeString);
}