Fixes#7919.
This PR addresses a TUI display bug where the "Worked for" separator
would appear prematurely during the planning stage.
**Changes:**
- Added `had_work_activity` flag to `ChatWidget` to track if actual work
(exec commands, MCP tool calls, patches) was performed in the current
turn.
- Updated `handle_streaming_delta` to only display the
`FinalMessageSeparator` if both `needs_final_message_separator` AND
`had_work_activity` are true.
- Updated `handle_exec_end_now`, `handle_patch_apply_end_now`, and
`handle_mcp_end_now` to set `had_work_activity = true`.
**Verification:**
- Ran `cargo test -p codex-tui` to ensure no regressions.
- Manual verification confirms the separator now only appears after
actual work is completed.
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Co-authored-by: Josh McKinney <joshka@openai.com>
