fix(session): restore sandbox-fs after agent-run quiesce

Agent runs quiesced stdio MCP extensions including sandbox-fs, but reconcile
skipped re-adding missing stdio extensions and treated absent listings as OK.
Preserve sandbox-fs across quiesce and re-add other required stdio extensions
on the next reconcile so generate_image and write_file stay available.

Co-authored-by: Cursor <cursoragent@cursor.com>

session-runtime-lifecycle.test.mjs

@@ -2,6 +2,7 @@ import assert from 'node:assert/strict';
 import test from 'node:test';
 import {
   cancelSessionActiveRequest,
+  PRESERVED_STDIO_EXTENSIONS,
   quiesceSessionStdioExtensions,
   sessionStdioExtensionNames,
 } from './session-runtime-lifecycle.mjs';
@@ -82,7 +83,7 @@ test('quiesceSessionStdioExtensions removes stdio children without deleting sess
   const result = await quiesceSessionStdioExtensions(apiFetch, 'session-1');
 
   assert.deepEqual(result, {
-    removed: ['sandbox-fs', 'tkmind-search'],
+    removed: ['tkmind-search'],
     skipped: false,
   });
   assert.deepEqual(
@@ -90,23 +91,27 @@ test('quiesceSessionStdioExtensions removes stdio children without deleting sess
     [
       '/sessions/session-1/extensions',
       '/agent/remove_extension',
-      '/agent/remove_extension',
     ],
   );
   assert.deepEqual(
     calls.slice(1).map(({ init }) => JSON.parse(init.body)),
     [
-      { session_id: 'session-1', name: 'sandbox-fs' },
       { session_id: 'session-1', name: 'tkmind-search' },
     ],
   );
+  assert.equal(PRESERVED_STDIO_EXTENSIONS.has('sandbox-fs'), true);
   assert.equal(calls.some(({ pathname }) => pathname.includes('delete')), false);
 });
 
 test('quiesceSessionStdioExtensions fails when upstream removal is not acknowledged', async () => {
   const apiFetch = async (pathname) => {
     if (pathname.endsWith('/extensions')) {
-      return jsonResponse({ extensions: [{ name: 'sandbox-fs', type: 'stdio' }] });
+      return jsonResponse({
+        extensions: [
+          { name: 'sandbox-fs', type: 'stdio' },
+          { name: 'tkmind-search', type: 'stdio' },
+        ],
+      });
     }
     return jsonResponse({ message: 'remove failed' }, { ok: false, status: 500 });
   };