TUI: use cumulative turn duration for worked-for separator

[etraut-openai]

Why

Fixes #19814.

The TUI's current Worked for ... timing behavior is a leftover from #9599. At that point, models could emit multiple assistant messages in one turn for preambles/commentary, but the TUI did not yet have a reliable signal that an assistant message was the final answer when it started streaming. To avoid showing an ever-growing elapsed time on each preamble separator, #9599 made the separator timer incremental by tracking elapsed time since the previous separator.

That workaround is no longer the right model for the final completed-turn display. Since then, #16638 added protocol-native turn timing, including duration_ms on turn completion. With that cumulative duration available at the point where the TUI renders the completed-turn separator, the UI can show the actual turn duration directly instead of carrying per-separator timing state.

What Changed

• Thread duration_ms into ChatWidget::on_task_complete from both legacy TurnCompleteEvent handling and app-server TurnCompleted notifications.
• Use duration_ms for the final Worked for ... separator, falling back to the status indicator timer only when the protocol duration is unavailable.
• Keep mid-turn separators before later assistant text as plain visual dividers instead of clocked Worked for ... separators.
• Remove the old incremental separator timer state and helper (last_separator_elapsed_secs / worked_elapsed_from).
• Add a snapshot regression test for a turn that runs a command and then completes with a final answer, verifying the final separator uses the cumulative turn duration.

Verification

• cargo test -p codex-tui final_worked_for_uses_cumulative_turn_duration_snapshot
• just fix -p codex-tui

Manual repro prompt:

Manual timing repro. First send a short preamble/commentary sentence before using tools. Then run exactly this shell command: sleep 75; echo MANUAL_TIMING_DONE. After the command finishes, give a final answer that says "done". Do not skip the preamble.

After this change, the mid-turn break before the final answer should be a plain divider, and the final completed-turn separator should show Worked for ... using the cumulative turn duration.

Before:

After:

[fcoury-oai]

I was able to replicate the issue on alpha codex and confirmed the fix works.