Add deterministic tests

frontend/deterministic-tests/src/utils/assert.ts

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+export function assert(value: boolean, message: string): asserts value {
+    if (!value) {
+        throw new Error(message);
+    }
+}

frontend/deterministic-tests/src/utils/find-free-port.ts

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+import * as net from "node:net";
+
+export interface PortReservation {
+    port: number;
+    release: () => void;
+}
+
+/**
+ * Find a free port and keep it reserved until the caller explicitly releases it.
+ */
+export async function findFreePort(): Promise<PortReservation> {
+    return new Promise((resolve, reject) => {
+        const server = net.createServer();
+        server.listen(0, "127.0.0.1", () => {
+            const addr = server.address();
+            if (addr === null || typeof addr === "string") {
+                server.close();
+                reject(new Error("Failed to get port from server"));
+                return;
+            }
+            const { port } = addr;
+            resolve({
+                port,
+                release: () => server.close()
+            });
+        });
+        server.on("error", reject);
+    });
+}

frontend/deterministic-tests/src/utils/sleep.ts

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+export async function sleep(ms: number): Promise<void> {
+    return new Promise((resolve) => setTimeout(resolve, ms));
+}

frontend/deterministic-tests/src/utils/with-timeout.ts

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+export async function withTimeout<T>(
+    promise: Promise<T>,
+    timeoutMs: number,
+    message: string
+): Promise<T> {
+    let timeoutId: ReturnType<typeof setTimeout> | undefined = undefined;
+    const timeoutPromise = new Promise<never>((_resolve, reject) => {
+        timeoutId = setTimeout(() => {
+            reject(new Error(message));
+        }, timeoutMs);
+    });
+    return Promise.race([promise, timeoutPromise]).finally(() => {
+        clearTimeout(timeoutId);
+    });
+}