Improve websocket liveness checks and idle timeout handling

- Idle timeout no longer requires receive activity alone: the send loop
  extends the idle deadline whenever it makes write progress
  (rate-limited to once per second).

- On idle timeout, initiate a proper close handshake carrying an
  "idle timeout" reason instead of abruptly tearing down the
  connection, so well-behaved clients can tell why they were
  disconnected. The connection is torn down if the handshake does not
  complete within a 10s grace period.

- Dead peers are instead detected at the transport layer: a new
  websocket.tcp-user-timeout option (default 2min, Linux only) applies
  TCP_USER_TIMEOUT to the listening socket, closing connections whose
  peer stops acknowledging sent data.

- The "websocket connection aborted" warning and ws_clients_aborted
  metric now fire only when the client actor task is actually aborted
  or panics, not on every normal disconnect.

- Fix the kick log message to report the channel's configured capacity
  instead of its remaining capacity (always 0 at that point).

Author: aasoni

Cargo.toml

@@ -298,7 +298,7 @@ similar = "2.3"
 slab = "0.4.7"
 sled = "0.34.7"
 smallvec = { version = "1.11", features = ["union", "const_generics"] }
-socket2 = "0.5"
+socket2 = { version = "0.5", features = ["all"] }
 sourcemap = "9.3.1"
 sqllogictest = "0.17"
 sqllogictest-engines = "0.17"