feat: TCP keepalive for Postgres and MySQL connections

[adriangb]

Adds TCP keepalive support, closing #3540. This is @xuehaonan27's #3559 rebased onto the current connect_tcp and reduced to an additive API; the design credit is theirs.

Why

Without keepalive, a connection whose server disappeared without closing the socket never finds out. A failover, a killed container, or a dropped NAT mapping leaves the client blocked reading a response that will never arrive. There is nothing left to retransmit, so no RST is ever provoked, and the read waits forever. TCP_NODELAY, which is all SQLx sets today, does not help, and a server-side statement_timeout cannot fire on a server that is gone.

We hit this in production: we have a ~ hot loop that is constantly querying so we are almost guaranteed to hit this every server restart (pgbouncer scale down in our case). It reproduces reliably by holding an ACCESS EXCLUSIVE lock so the query blocks server-side, then removing the server from the network and restarting it. Notably a restart w/o an in-flight query won't reproduce: the query has to be already acknowledged and executing, which is exactly the common case during a real failover.

@abonander, in #3559 (comment) you allowed for this case: "I suppose that's still preferable to it hanging forever on a read that will never complete." One correction on the objection raised there, that a keepalive timeout is only noticed the next time we use the socket: that is not true for a blocked reader. When the probes are exhausted the kernel sets sk_err to ETIMEDOUT and wakes anyone parked on the socket, so a pending read fails rather than waiting for someone to poke it. That is precisely the case that hangs today.

Since #3559 was opened, three other users have reported the same silent-stale-connection failure in it (Aug 2025, Dec 2025, Feb 2026), and the author of the follow-up #4159 moved to tokio-postgres over it.

What this adds

• sqlx_core::net::TcpKeepalive, with idle / interval / retries, cfg-gated for the platforms where socket2 does not support the latter two.
• net::connect_tcp_with_keepalive. connect_tcp keeps its signature and delegates with None, so external drivers (Compile-time support for external drivers #3889) are unaffected.
• PgConnectOptions::tcp_keepalive() and MySqlConnectOptions::tcp_keepalive().
• libpq-compatible URL parameters for Postgres: keepalives, keepalives_idle, keepalives_interval, keepalives_count. These previously hit the ignoring unrecognized connect parameter branch, so anyone copying a libpq DSN silently got no keepalive.

Keepalive stays off by default, so behaviour is unchanged for existing users. (libpq defaults it on with a 2h idle; I did not want to change a default in this PR, but happy to if you would prefer parity.)

Tests

Unit tests for the builder and for the URL parameters, including that keepalives=0 disables it again after another parameter turned it on. The setsockopt itself is not unit-testable without a socket; it is exercised by every connection made with keepalive configured.

I am carrying this as a patched fork in the meantime, so I am happy to iterate on the API shape or split MySQL out if that makes review easier.