test: add shared mock infrastructure and route tests (phase 7)

Consolidate duplicated MockSession/MockStateStore into test/mocks/,
migrate respawn tests to shared mocks, and add 58 route tests for
session, system, and respawn endpoints using Fastify app.inject().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/mocks/test-helpers.ts

@@ -0,0 +1,37 @@
+/**
+ * Reusable async test helpers.
+ * Extracted from respawn-test-utils.ts.
+ */
+
+/** Wait for an EventEmitter to emit a specific event, with timeout */
+export function waitForEvent(
+  emitter: { once: (event: string, listener: (...args: unknown[]) => void) => void },
+  event: string,
+  timeoutMs = 5000,
+): Promise<unknown> {
+  return new Promise((resolve, reject) => {
+    const timer = setTimeout(
+      () => reject(new Error(`Timed out waiting for event "${event}" after ${timeoutMs}ms`)),
+      timeoutMs,
+    );
+    emitter.once(event, (...args: unknown[]) => {
+      clearTimeout(timer);
+      resolve(args.length === 1 ? args[0] : args);
+    });
+  });
+}
+
+/** Create a deferred promise with external resolve/reject */
+export function createDeferred<T = void>(): {
+  promise: Promise<T>;
+  resolve: (value: T) => void;
+  reject: (reason?: unknown) => void;
+} {
+  let resolve!: (value: T) => void;
+  let reject!: (reason?: unknown) => void;
+  const promise = new Promise<T>((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+  return { promise, resolve, reject };
+}