The May 27 merge of origin/main into bb/gui re-introduced two callers of
_content_display_text (in _inflight_text and _history_to_messages) but
dropped the helper definition itself, leaving an unresolved reference.
NameError fires on every user message via _start_inflight_turn ->
_inflight_text, taking down both the TUI and the desktop (which share
this gateway backend) the moment input is dispatched.
Restores the helper verbatim from main (commit 36c99af37) -- pure
structured-content text extractor, no other dependencies.