Handle orphan exec ends without clobbering active exploring cell (#12313)

Summary
- distinguish exec end handling targets (active tracking, active orphan
history, new cell) so unified exec responses don’t clobber unrelated
exploring cells
- ensure orphan ends flush existing exploring history when complete,
insert standalone history entries, and keep active cells correct
- add regression tests plus a snapshot covering the new behavior and
expose the ExecCell completion result for verification

Fix for https://github.com/openai/codex/issues/12278

---------

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

codex-rs/tui/src/exec_cell/model.rs

@@ -1,3 +1,10 @@
+//! Data model for grouped exec-call history cells in the TUI transcript.
+//!
+//! An `ExecCell` can represent either a single command or an "exploring" group of related read/
+//! list/search commands. The chat widget relies on stable `call_id` matching to route progress and
+//! end events into the right cell, and it treats "call id not found" as a real signal (for
+//! example, an orphan end that should render as a separate history entry).
+
 use std::time::Duration;
 use std::time::Instant;
 
@@ -67,17 +74,24 @@ impl ExecCell {
         }
     }
 
+    /// Marks the most recently matching call as finished and returns whether a call was found.
+    ///
+    /// Callers should treat `false` as a routing mismatch rather than silently ignoring it. The
+    /// chat widget uses that signal to avoid attaching an orphan `exec_end` event to an unrelated
+    /// active exploring cell, which would incorrectly collapse two transcript entries together.
     pub(crate) fn complete_call(
         &mut self,
         call_id: &str,
         output: CommandOutput,
         duration: Duration,
-    ) {
-        if let Some(call) = self.calls.iter_mut().rev().find(|c| c.call_id == call_id) {
-            call.output = Some(output);
-            call.duration = Some(duration);
-            call.start_time = None;
-        }
+    ) -> bool {
+        let Some(call) = self.calls.iter_mut().rev().find(|c| c.call_id == call_id) else {
+            return false;
+        };
+        call.output = Some(output);
+        call.duration = Some(duration);
+        call.start_time = None;
+        true
     }
 
     pub(crate) fn should_flush(&self) -> bool {