clear running commands in various places (#2325)

we have a very unclear lifecycle for the chatwidget—this should only
have to be added in one place! but this fixes the "hanging commands"
issue where the active_exec_cell wasn't correctly cleared when commands
finished.

To repro w/o this PR:
1. prompt "run sleep 10"
2. once the command starts running, press <kbd>Esc</kbd>
3. prompt "run echo hi"

Expected: 

```
✓ Completed
  └ ⌨️ echo hi

codex
hi
```

Actual:

```
⚙︎ Working
  └ ⌨️ echo hi

▌ Ask Codex to do anything
```

i.e. the "Working" never changes to "Completed".

The bug is fixed with this PR.

codex-rs/tui/src/chatwidget.rs

@@ -182,6 +182,7 @@ impl ChatWidget<'_> {
         }
         // Mark task stopped and request redraw now that all content is in history.
         self.bottom_pane.set_task_running(false);
+        self.running_commands.clear();
         self.mark_needs_redraw();
     }
 
@@ -198,6 +199,7 @@ impl ChatWidget<'_> {
     fn on_error(&mut self, message: String) {
         self.add_to_history(&history_cell::new_error_event(message));
         self.bottom_pane.set_task_running(false);
+        self.running_commands.clear();
         self.stream.clear_all();
         self.mark_needs_redraw();
     }
@@ -473,6 +475,7 @@ impl ChatWidget<'_> {
     fn interrupt_running_task(&mut self) {
         if self.bottom_pane.is_task_running() {
             self.active_exec_cell = None;
+            self.running_commands.clear();
             self.bottom_pane.clear_ctrl_c_quit_hint();
             self.submit_op(Op::Interrupt);
             self.bottom_pane.set_task_running(false);