Previously, there was a `StringRef.copy` method which would copy the string into
the given buffer. However, it was not defined for the case when the buffer was
too small. It moved the responsibility of making sure the buffer is large enough
to the caller.
Unfortunately, in practice that easily hides bugs in builds without asserts
which don't come up in testing much. Now, the method is replaced with
`StringRef.copy_utf8_truncated` which has much more well defined semantics and
also makes sure that the string remains valid utf-8.
This also renames `unsafe_copy` to `copy_unsafe` to make the naming more similar
to `copy_utf8_truncated`.
Pull Request: https://projects.blender.org/blender/blender/pulls/133677
e1753900b7