script: make Node::xml_serialize fallible. (#38532)

Testing: [Try run][1] did not reveal any test failures. There doesn't
seem to be any straightforward failure scenarios that can be triggered
in `xml5ever` that are not IO errors and the xml_serialize method simply
serializes to a String buffer.

[1]:
https://github.com/servo/servo/actions/runs/16824267959/job/47657275606l

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/script/dom/shadowroot.rs

@@ -464,6 +464,8 @@ impl ShadowRootMethods<crate::DomTypeHolder> for ShadowRoot {
         // algorithm steps with this and true.
         self.upcast::<Node>()
             .fragment_serialization_algorithm(true, can_gc)
+            .inspect_err(|error| warn!("fragment serialization failed: {error:?}"))
+            .unwrap_or_default()
     }
 
     /// <https://html.spec.whatwg.org/multipage/#dom-shadowroot-innerhtml>