Expose retained command sessions in incremental world state (#59633)

## Why

A Code Mode script can start a command without returning its session handle. Make retained sessions visible to the model so it can find running commands and collect exited results with `write_stdin`.

## What changed

- Add the disabled-by-default `command_sessions_world_state` feature.
- Publish snapshots of up to 16 retained sessions with their IDs, running or exited status, and whether they are in use. Report omitted sessions and suppress unchanged updates.
- Preserve session visibility when `write_stdin` is unavailable and refresh snapshots after compaction. Clear stale session handles after restart or when the feature is disabled.

## Testing

Add an end-to-end scenario and model-visible history snapshot covering a dropped command handle, unchanged state, unavailable polling, compaction, exit and result collection, and stale-handle cleanup after restart.

GitOrigin-RevId: 45704d8c6403149997231aafb8496c12879aff23

codex-rs/core/src/unified_exec/command_sessions.rs

@@ -0,0 +1,29 @@
+//! Read-only snapshots of retained command sessions, including uncollected exits.
+//! Handles stay private until their initial exec_command collection finishes.
+
+use super::UnifiedExecProcessManager;
+use crate::context::world_state::CommandSession;
+use crate::context::world_state::CommandSessionStatus;
+use crate::context::world_state::CommandSessionsState;
+use std::sync::atomic::Ordering;
+
+pub(crate) async fn command_sessions_state(
+    manager: &UnifiedExecProcessManager,
+) -> CommandSessionsState {
+    let store = manager.process_store.lock().await;
+    CommandSessionsState::new(
+        store
+            .processes
+            .values()
+            .filter(|entry| !entry.initial_exec_command_active.load(Ordering::Acquire))
+            .map(|entry| CommandSession {
+                session_id: entry.process_id,
+                status: if entry.process.has_exited() {
+                    CommandSessionStatus::Exited
+                } else {
+                    CommandSessionStatus::Running
+                },
+                in_use: entry.process.interaction_lock().try_lock_owned().is_err(),
+            }),
+    )
+}

codex-rs/core/src/unified_exec/mod.rs

@@ -44,6 +44,7 @@ use crate::tools::network_approval::DeferredNetworkApproval;
 use codex_core_plugins::PluginMetricsSidecar;
 
 mod async_watcher;
+mod command_sessions;
 mod errors;
 mod oneshot;
 pub(crate) mod output;
@@ -58,6 +59,7 @@ pub(crate) fn set_deterministic_process_ids_for_tests(enabled: bool) {
     process_manager::set_deterministic_process_ids_for_tests(enabled);
 }
 
+pub(crate) use command_sessions::command_sessions_state;
 pub(crate) use errors::UnifiedExecError;
 pub(crate) use process::UnifiedExecProcess;
 pub(crate) use stdin_approval::TerminalPermissions;