feat: add WebSocket heartbeat ping/pong with configurable interval and max-missed

[Copilot]

Summary

Implements the heartbeat (ping/pong) feature for the WebSocket tunnel between the expose-rs client and server.

Both sides send a WebSocket Ping frame periodically. If no Pong is received for too many consecutive pings, the connection is closed. The client retries after closure; the server only cleans up resources.

Changes

expose-common

• Added HeartbeatConfig struct with:
  • interval: Duration — how often to send a ping (default: 15 s, configurable via CLI)
  • max_missed: u32 — how many consecutive missed pongs before closing (default: 2, configurable via CLI)
• Added constants DEFAULT_HEARTBEAT_INTERVAL_SECS and DEFAULT_HEARTBEAT_MAX_MISSED
• All new items are re-exported from both expose_client and expose_server

expose-client

• Changed the WS writer channel from mpsc::UnboundedSender<Vec<u8>> to mpsc::UnboundedSender<Message> so that both binary frames and Ping control frames flow through the same writer task
• Spawns a heartbeat timer task in run_client that sends pings every interval and closes the connection after max_missed consecutive missed pongs
• Handles Message::Pong in the receive loop to reset the missed-pong counter
• Updated run_client_once_with_channel_config to accept HeartbeatConfig; run_client_once uses defaults
• Added --heartbeat-interval and --heartbeat-max-missed CLI arguments (with defaults 15 / 2)

expose-server

• Same channel-type change, heartbeat task, pong handling, and signature update as the client
• run_server_with_channel_config now accepts HeartbeatConfig; run_server uses defaults
• Added --heartbeat-interval and --heartbeat-max-missed CLI arguments

Integration tests

• Updated all call sites to pass HeartbeatConfig::default() (no behaviour change for existing tests)
• Added test_heartbeat_closes_dead_tunnel: verifies that when the client stops sending pongs, the server detects the timeout and closes the tunnel, and that a new client can reconnect cleanly afterwards

Testing

All 15 integration tests and 20 unit tests pass, including the new heartbeat test.