_convert_user_message hand-inlined the same blank-text-filter +
cache_control-relocation + placeholder-fallback logic that
_fix_blank_text_blocks_in_list (added in the cherry-picked commit)
implements as a reusable helper. Replace the inline copy with a call
to the helper, eliminating ~35 lines of duplication.
Follow-up fix on top of PR #77134 by @pooyan6.
kshitij