E2E test: Heartbeat handling — verify echo and callback

[s2x]

Closes #161

Problem

StreamConnection::dispatchServerPush() handles heartbeats (key 0x0017) by echoing back a heartbeat frame and optionally invoking a registered callback. No E2E test verifies this behavior. In production, heartbeat failures cause connection drops.

Expected test

public function testHeartbeatIsEchoedBack(): void
{
    // 1. Connect with a short heartbeat interval (e.g., 2 seconds)
    //    by sending TuneResponseV1 with heartbeat=2
    // 2. Register heartbeat callback
    // 3. Wait in readLoop for heartbeat interval + margin
    // 4. Verify callback was invoked
    // 5. Verify connection is still alive (heartbeat was echoed)
}

Why it matters

• Heartbeat failures are the ci: add GitHub Actions workflow for unit and E2E tests #1 cause of unexpected connection drops in production
• The echo mechanism was previously buggy (issue Bug: Heartbeat echo via sendMessage() causes correlationId desynchronization #101 — correlationId desync, now fixed)
• Regression protection for the heartbeat path is critical

Notes

• This test requires waiting for the heartbeat interval, so it will be slower
• Consider using a short heartbeat (1-2 seconds) to keep test duration reasonable
• The Tune negotiation determines the heartbeat interval

Acceptance criteria

• Heartbeat callback is invoked when server sends heartbeat
• Connection remains alive after heartbeat exchange
• No correlationId desync after heartbeat (regression test for Bug: Heartbeat echo via sendMessage() causes correlationId desynchronization #101)