[FIX] web_editor: change do not bold nobold after bold

If we have a content such as:

`aaa <B>bbb</B> ccc`

and you eg. change size of `a bbb c`, the c gets bold because of an
issue in the merging after style is applied.

reason: we check that the style was same as previous nodes, but because
walkpoints text[a]->B->text[bbb]->B->text[c] is expurged of duplicates
it becomes text[a]->B->text[bbb]->text[c] so we erroneoulsy think that
the node before text[c] is text[bbb] (instead of B tag)

note: 12.5 forward-port (skipping [12.3,12.4]) of 11.0 #37254 (also
contain #37345)

opw-2071930
closes #37295

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>