test(runtime-host): cover SessionAdmissionGate.detach

The behaviour change was listed nowhere and had no test: work detached
from an admission must take admissions of its own, queued behind the
active one like any other caller.

Generated-by: Claude Code

Author: Astro-Han

packages/runtime-host/src/__tests__/session-admission-gate.test.ts

@@ -147,3 +147,28 @@ test('rejects accidental admission re-entry instead of deadlocking', async () =>
     );
   });
 });
+
+test('work detached from an admission takes admissions of its own', async () => {
+  const gate = new SessionAdmissionGate();
+  const release = deferred();
+  const order: string[] = [];
+  let detached!: Promise<void>;
+
+  // The detached work starts inside the admission and admits before the
+  // admission ends, which is the order a drained Turn reaches its first
+  // admission in. Inherited context would reject it as re-entry.
+  await gate.run('session', async () => {
+    order.push('active:start');
+    detached = gate.detach(async () => {
+      await gate.run('session', () => {
+        order.push('detached:admitted');
+      });
+    });
+    await Promise.resolve();
+    order.push('active:end');
+    release.resolve();
+  });
+  await release.promise;
+  await detached;
+  assert.deepEqual(order, ['active:start', 'active:end', 'detached:admitted']);
+});