Box tree construction: only unset pseudo elements if the box is rebuilt (#39930)

Box Tree re-construction may cause the loss of pseudo element:
https://github.com/servo/servo/issues/39225#issuecomment-3404462089,
then causes transition stucks.
We should only unset  pseudo elements if the box is rebuilt.

Testing: fix manual test in https://github.com/servo/servo/issues/39225,
CI reports no regression:
[action](https://github.com/longvatrong111/servo/actions/runs/18572401080)
Fixes: https://github.com/servo/servo/issues/39225

cc: @xiaochengh

---------

Signed-off-by: batu_hoang <longvatrong111@gmail.com>
Signed-off-by: batu_hoang <hoang.binh.trong@huawei.com>

tests/wpt/mozilla/meta/MANIFEST.json

@@ -12672,6 +12672,15 @@
        {}
       ]
      ],
+     "pseudo-element-transition-continues-on-box-tree-re-construction.html": [
+      "131c49a9780ec215a8cf8964d148473f85207dd0",
+      [
+       null,
+       {
+        "testdriver": true
+       }
+      ]
+     ],
      "transition-events.html": [
       "b561fc8353276e6bdd13a9d1b965f57733ecd19b",
       [