String move constructor and move assignment don't move

The String::String(String&&) and String::operator=(String&&) do not perform expected operation i.e. they don't move, but instead they copy.

No file or test is named. Locate the String class declarations and definitions for String::String(String&&) and String::operator=(String&&), then inspect the existing tests or add a focused regression test. Done means both operations have move semantics rather than copying, with the expected behavior covered by tests.

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