Make sure we consider buffer length when marshalling back Unicode ByValTStr fields...

Make sure we consider buffer length when marshalling back Unicode ByValTStr fields (#54695)

* Use string constructor that takes length instead of the one that searches for a null terminator.

Fixes #54662

* Marshal back buffer size or string to first null terminator, whichever is shorter

* Add tests.

* Add unicode test.

* Use the same implementation style for the wstr case case as the cstr case

* Fix accidental deletion from test.