test: add server-side negative test harness (#249)

Adds src/scenarios/server/negative.test.ts with spawn helpers. Wires
two cases: the previously-orphaned no-dns-rebinding-protection.ts and
a new sep-2164-empty-contents.ts. Strengthens the AGENTS.md guidance
to point at the harness.

src/scenarios/server/negative.test.ts

@@ -0,0 +1,109 @@
+import { spawn, ChildProcess } from 'child_process';
+import path from 'path';
+import { DNSRebindingProtectionScenario } from './dns-rebinding';
+import { ResourcesNotFoundErrorScenario } from './resources';
+
+function startServer(scriptPath: string, port: number): Promise<ChildProcess> {
+  return new Promise((resolve, reject) => {
+    const isWindows = process.platform === 'win32';
+    const proc = spawn('npx', ['tsx', scriptPath], {
+      env: { ...process.env, PORT: port.toString() },
+      stdio: ['ignore', 'pipe', 'pipe'],
+      shell: isWindows
+    });
+    let stderr = '';
+    proc.stderr?.on('data', (d) => (stderr += d.toString()));
+    const timeout = setTimeout(() => {
+      proc.kill('SIGKILL');
+      reject(
+        new Error(`Server ${scriptPath} failed to start within 30s: ${stderr}`)
+      );
+    }, 30000);
+    proc.stdout?.on('data', (data) => {
+      if (data.toString().includes('running on')) {
+        clearTimeout(timeout);
+        resolve(proc);
+      }
+    });
+    proc.on('error', (err) => {
+      clearTimeout(timeout);
+      reject(err);
+    });
+  });
+}
+
+function stopServer(proc: ChildProcess | null): Promise<void> {
+  return new Promise((resolve) => {
+    if (!proc || proc.killed) return resolve();
+    const t = setTimeout(() => {
+      proc.kill('SIGKILL');
+      resolve();
+    }, 5000);
+    proc.once('exit', () => {
+      clearTimeout(t);
+      resolve();
+    });
+    proc.kill('SIGTERM');
+  });
+}
+
+describe('Server scenario negative tests', () => {
+  describe('dns-rebinding-protection', () => {
+    let serverProcess: ChildProcess | null = null;
+    const PORT = 3004;
+
+    beforeAll(async () => {
+      serverProcess = await startServer(
+        path.join(
+          process.cwd(),
+          'examples/servers/typescript/no-dns-rebinding-protection.ts'
+        ),
+        PORT
+      );
+    }, 35000);
+
+    afterAll(async () => {
+      await stopServer(serverProcess);
+    });
+
+    it('emits FAILURE against a server without rebinding protection', async () => {
+      const scenario = new DNSRebindingProtectionScenario();
+      const checks = await scenario.run(`http://localhost:${PORT}/mcp`);
+
+      const rebindingCheck = checks.find(
+        (c) => c.id === 'localhost-host-rebinding-rejected'
+      );
+      expect(rebindingCheck?.status).toBe('FAILURE');
+    }, 10000);
+  });
+
+  describe('sep-2164-resource-not-found', () => {
+    let serverProcess: ChildProcess | null = null;
+    const PORT = 3005;
+
+    beforeAll(async () => {
+      serverProcess = await startServer(
+        path.join(
+          process.cwd(),
+          'examples/servers/typescript/sep-2164-empty-contents.ts'
+        ),
+        PORT
+      );
+    }, 35000);
+
+    afterAll(async () => {
+      await stopServer(serverProcess);
+    });
+
+    it('emits FAILURE for no-empty-contents and WARNING for error-code against a server returning empty contents', async () => {
+      const scenario = new ResourcesNotFoundErrorScenario();
+      const checks = await scenario.run(`http://localhost:${PORT}/mcp`);
+
+      const noEmpty = checks.find((c) => c.id === 'sep-2164-no-empty-contents');
+      expect(noEmpty?.status).toBe('FAILURE');
+
+      const errorCode = checks.find((c) => c.id === 'sep-2164-error-code');
+      expect(errorCode?.status).toBe('WARNING');
+    }, 10000);
+  });
+});