fix(tui): only show 'Worked for' separator when actual work was performed (#8958)

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.

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

codex-rs/tui/src/chatwidget/tests.rs

@@ -438,6 +438,7 @@ async fn make_chatwidget_manual(
         is_review_mode: false,
         pre_review_token_info: None,
         needs_final_message_separator: false,
+        had_work_activity: false,
         last_rendered_width: std::cell::Cell::new(None),
         feedback: codex_feedback::CodexFeedback::new(),
         current_rollout_path: None,