test: cover the readAttachment and hostDescription bench stubs directly

packages/client/test-runtime/tests/runtime.spec.tsx

@@ -467,9 +467,12 @@ describe('fixture session face', () => {
     await runtime.sessions.add({ id: 's1' })
     const bare = runtime.sessions.behavior('s1')
     expect(() => bare.prompt()).toThrow(/prompt is not stubbed/)
+    expect(() => bare.readAttachment('att-1' as Parameters<typeof bare.readAttachment>[0])).toThrow(/readAttachment is not stubbed/)
     expect(() => bare.cancel()).toThrow(/cancel is not stubbed/)
     expect(() => bare.command()).toThrow(/command is not stubbed/)
     expect(() => bare.loadOlder()).toThrow(/loadOlder is not stubbed/)
+    // No host handshake exists in the bench unless a fixture supplies one.
+    expect(runtime.sessions.hostDescription()).toBeUndefined()
     await runtime.dispose()
   })