Modopt/modreq parsing is busted in some cases for `UnsafeAccessor` scenarios

[AaronRobinsonMSFT]

Issue #109665 was fixed with #109694 and assumed to also address issues with readonly and so tests were added with in #109944 - these tests do pass.

The issue is best showcased with the following:

static class Accessors
{
    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "<obj>P")]
    public static extern ref readonly object MEMBER(ref readonly Foo value); 
    
    public static void A()
    {
        [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "<obj>P")]
        static extern ref readonly object LOCAL(ref readonly Foo value);
    }
}

The MEMBER and LOCAL are the same definition, but MEMBER uses an modreq when LOCAL doesn't. For testing the LOCAL scenario was used for validation and this created a false sense of correctness in the tests. The result is the modopt/modreq issues can still occur for fields depending on how the UnsafeAccessor is defined. This means we need to service .NET 9 again, see #109709, and make the correct fix in .NET 10 along with updating the testing so we validate both types of definition.

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