Fix IndexOutOfRangeException when an Enum's MetadataToken collides with Nullable<T>

[SergeiPavlov]

Copy of DataObjects-NET#472

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Note

Medium Risk
Changes core packed-tuple field accessor resolution; behavior is narrowed to fix a rare token collision bug, with regression tests covering the edge case.

Overview
Fixes IndexOutOfRangeException during TupleDescriptor.Create when a tuple field type is an enum whose MetadataToken matches Nullable<T> but lives in a different module.

In TupleLayout.ValueFieldAccessorResolver.GetValue, nullable unwrap for enum underlying-type resolution now requires both the known Nullable<> metadata token and the same module as WellKnownTypes.NullableOfT, instead of token alone. That stops non-generic enums from being treated like Nullable<> and calling GetGenericArguments()[0] on them.

Adds NullableTokenCollisionTest, which emits filler types so a dynamic enum shares Nullable<>’s token in another assembly, and asserts tuple descriptor creation for the plain enum and for Nullable<that enum> both succeed.

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