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/routes/_route-test-utils.ts

@@ -0,0 +1,40 @@
+/**
+ * Shared utilities for route testing.
+ *
+ * Creates minimal Fastify instances with just the route module under test
+ * and a mock context. Uses app.inject() for HTTP testing without real ports.
+ */
+import Fastify, { type FastifyInstance } from 'fastify';
+import fastifyCookie from '@fastify/cookie';
+import { createMockRouteContext, type MockRouteContext } from '../mocks/index.js';
+
+export interface RouteTestHarness {
+  app: FastifyInstance;
+  ctx: MockRouteContext;
+}
+
+/**
+ * Creates a Fastify instance with a route module registered against a mock context.
+ *
+ * @param registerFn - The route registration function (e.g., registerSessionRoutes).
+ *   Uses `any` for ctx parameter because route functions expect typed port intersections
+ *   that MockRouteContext satisfies structurally but not nominally.
+ * @param ctxOptions - Optional overrides for the mock context
+ */
+export async function createRouteTestHarness(
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  registerFn: (app: FastifyInstance, ctx: any) => void,
+  ctxOptions?: { sessionId?: string },
+): Promise<RouteTestHarness> {
+  const app = Fastify({ logger: false });
+
+  // Register cookie plugin — some routes access req.cookies
+  await app.register(fastifyCookie);
+
+  const ctx = createMockRouteContext(ctxOptions);
+
+  registerFn(app, ctx);
+  await app.ready();
+
+  return { app, ctx };
+}