test(desktop): fail promptly when synthetic children never get ready

The fake application now leaves a marker when its children never report
ready, and the tests' session requests throw on it, so capture releases the
children and the test fails at once instead of waiting for its deadline.

Refs #10366

Co-Authored-By: gensokyo-dev <342580529+gensokyo-dev@users.noreply.github.com>

src/application/desktop/DesktopApplicationCapture.test.ts

@@ -145,13 +145,19 @@ const installProcessFamily = async (root: string, path: string) => {
       `setsid sh -c 'trap "touch ${join(root, "setsid-term")}; exit 0" TERM; touch "${join(root, "setsid-ready")}"; ${loop}' &`,
       // Wait for both children to install their traps before writing state.ini.
       // Fail loudly rather than continue when a child never reports ready.
-      `ready=; for i in $(seq 1 500); do if [ -f "${join(root, "group-ready")}" ] && [ -f "${join(root, "setsid-ready")}" ]; then ready=1; break; fi; sleep 0.01; done; [ -n "$ready" ] || { echo "synthetic children did not report ready" >&2; exit 97; }`,
+      `ready=; for i in $(seq 1 500); do if [ -f "${join(root, "group-ready")}" ] && [ -f "${join(root, "setsid-ready")}" ]; then ready=1; break; fi; sleep 0.01; done; [ -n "$ready" ] || { echo "synthetic children did not report ready" >&2; touch "${join(root, "children-not-ready")}"; exit 97; }`,
       'printf "count=1\\n" > "$HOME/.config/fakeapp/state.ini"',
       loop,
     ].join("\n"),
   );
 };
 
+/** Surface a fake application whose children never got ready, instead of waiting. */
+const failIfChildrenNotReady = (root: string): void => {
+  if (fileExists(join(root, "children-not-ready")))
+    throw new Error("synthetic children did not report ready");
+};
+
 const host = (
   environment: NodeJS.ProcessEnv,
   nextRequest: () => SessionRequest | undefined = () => undefined,
@@ -660,6 +666,7 @@ describe.skipIf(process.platform !== "linux")(
       const statePath = join(home, ".config/fakeapp/state.ini");
       let writtenAt: number | undefined;
       const nextRequest = (): SessionRequest | undefined => {
+        failIfChildrenNotReady(root);
         // Wait for both children to install their traps before issuing the stop.
         if (readFileSyncSafe(statePath) !== "count=1\n") return undefined;
         if (
@@ -693,10 +700,12 @@ describe.skipIf(process.platform !== "linux")(
       await installProcessFamily(root, environment.PATH);
       const statePath = join(home, ".config/fakeapp/state.ini");
       const output = join(root, "out");
-      const nextRequest = (): SessionRequest | undefined =>
-        readFileSyncSafe(statePath) === "count=1\n"
+      const nextRequest = (): SessionRequest | undefined => {
+        failIfChildrenNotReady(root);
+        return readFileSyncSafe(statePath) === "count=1\n"
           ? { kind: "checkpoint", name: "written" }
           : undefined;
+      };
 
       const failed = await captureDesktopApplication(request(output), {
         ...host(environment, nextRequest),