server: public ConnectionInfo; Server::serve_connection returning ConnectionClosed

[rpb-ant]

Before this, owning the accept step meant raw hyper, which drops every timeout, retirement and drain guarantee. After:

let mut info = ConnectionInfo::new().with_peer_addr(peer).with_peer_certs(chain);
info.extensions_mut().insert(tenant); // cloned into every request
let closed = server.serve_connection(stream, info, drain.clone().wait()).await;
metrics.record(closed.reason()); // Closed | Shutdown | MaxAge | Idle | MaxRequests | Error

• #[non_exhaustive] ConnectionInfo, formerly the private PeerInfo, also at the crate root. PeerAddr / PeerCerts on requests always come from its peer fields, never from the extensions.
• Server::serve_connection(&self, io, info, shutdown) -> impl Future<Output = ConnectionClosed> + Send + 'static serves one already-accepted stream with the server's service and connection settings, on whichever runtime polls the future. Server::with_tls does not apply here: the caller completes any TLS handshake first.
• ConnectionClosed::reason() -> CloseReason (#[non_exhaustive]). Once shutdown or a retirement trigger tells a connection to wind down, that reason is final: a peer that errors mid-drain, or that sent nothing before shutdown, reports the wind-down reason, not Error.

First of four stacked PRs: #306 → #307 → #308 → #304. Part of #191 and #49.

[iainmcgin]

[claude code] Approving under the maintainers' agreement to review and merge each other's PRs. The added fix commits went through code, API, doc and deslop review; CI, server conformance and the MSRV check pass.