fix(tests): resolve 4 post-phase test failures

- platformClients.integration: iOS/Android push tests lacked
  setStatus() call before listNodes(), so platform filter excluded
  nodes. Added publishHeartbeat() to set platform on connection state.
- server.test: agent.send now emits run_state events before done (Phase
  1). Added sendAndWaitForDone() helper and updated test to find done
  event rather than assuming index 0.
- handlers.test: updated agent.send/cancel assertions to use find()
  and pass send arg to agent.cancel, consistent with run_state events.
- httpBody: req.destroy() closed socket before 413 response could be
  sent. Removed socket destruction from body reader; 413 responses now
  send Connection: close so Node closes the connection cleanly.

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

src/companion/platformClients.integration.test.ts

@@ -396,6 +396,7 @@ describe('platform clients integration', () => {
 
     try {
       await client.register();
+      await client.publishHeartbeat();
       const push = await client.registerPushToken({
         token: 'd'.repeat(64),
         topic: 'dev.flynn.ios',
@@ -424,6 +425,7 @@ describe('platform clients integration', () => {
 
     try {
       await client.register();
+      await client.publishHeartbeat();
       const push = await client.registerPushToken('e'.repeat(64));
 
       expect(push.updated).toBe(true);