xml: preserve DOCTYPE declaration when writing XML files (#12148)
* fix(xml): preserve DOCTYPE declaration when writing XML files
Pass `doctype=tree.docinfo.doctype` to all `ElementTree.write()` calls
so lxml does not silently drop the DOCTYPE on serialization. Also replace
`etree.tostring()` with BytesIO+write() in the diff and xmlstring paths
for consistency.
Fixes#2762
* test(xml): add integration test for DOCTYPE preservation
* feat(changelog): add fragment for xml DOCTYPE fix (#12148)
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(cherry picked from commit 5004c9f70f)
Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
