tree data: duplicate anydata children into target context

[Yeaseen]

Summary

Fix a use-after-free in cross-context duplication of data trees containing
anydata XML subtrees.

This addresses GHSA-jq97-j23h-2vqw:
GHSA-jq97-j23h-2vqw

Problem

lyd_dup_siblings_to_ctx() is expected to duplicate a data tree into the target
context. However, when duplicating an anydata node that stores its value as a
child subtree, the embedded children were copied with lyd_dup_siblings().

That preserves references to the source context. If the source tree and source
context are freed after successful duplication, later operations on the duplicate
can access freed memory. One affected path is XPath traversal through
lyd_trim_xpath().

Fix

Duplicate anydata child subtrees with lyd_dup_siblings_to_ctx() and
LYD_CTX(trg) so embedded children are copied into the same target context as
the containing anydata node.

Add a regression test that duplicates an anydata XML subtree into a second
context, destroys the original context, and then evaluates XPath on the
duplicate.

Verification

I ran the advisory reproducer against an ASan/UBSan build with this fix applied.
The reproducer completes successfully and no use-after-free is reported.

[michalvasko]

Already fixed in the devel branch but if you rebase (remove the fix), I can still merge the test.

[Yeaseen]

@michalvasko Done, thanks. I rebased onto devel, removed the duplicate fix, and kept only the regression test.