code-mode: make session shutdown authoritative (#29287)

## Summary

- Give each session and cell a hierarchical cancellation token.
- Track cell tasks so shutdown waits for admitted actors without polling
the registry.
- Make shutdown authoritative across concurrent admission and
non-cooperative callbacks.

## Why

A best-effort registry scan can miss cells admitted concurrently or
blocked behind the registry lock.

## Impact

Session shutdown reliably stops every admitted cell and rejects new work
once shutdown begins.

## Validation

- Stack-tip validation: `just test -p codex-code-mode -p
codex-code-mode-protocol` (70 passed).
- Parent branch: `cconger/code-mode-runtime-compact-03c-terminal-state`.

codex-rs/code-mode/src/service_tests.rs

@@ -360,7 +360,7 @@ await Promise.all([
         }
     );
 
-    tokio::time::sleep(Duration::from_millis(1100)).await;
+    tokio::time::sleep(Duration::from_secs(2)).await;
 
     let resumed_response = tokio::time::timeout(
         Duration::from_secs(1),