feat: extend TCP keepalive support to all TCP-based listeners and dialers

Add configurable TCP keepalive (keepalive, keepalive.idle,
keepalive.interval, keepalive.count) to all TCP-based transport types:
tls, mtls, mtcp, ws/wss, mws/mwss, http2, ssh, sshd listeners and
tls, mtls, mtcp, ws/wss, mws/mwss, ssh, sshd dialers.

For types that already use "keepalive" for an app-level protocol
(grpc listener/dialer, ws/mws dialers, ssh/sshd dialers), the TCP
keepalive parameters are exposed under the tcp.keepalive.* prefix to
avoid ambiguity.

Also extract shared helpers into internal/net/keepalive.go:
- WrapKeepaliveListener: applies KeepAliveConfig on every accepted TCPConn
- ApplyKeepalive: applies KeepAliveConfig to a single TCPConn

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dialer/ssh/dialer.go

@@ -9,6 +9,7 @@ import (
 	"github.com/go-gost/core/dialer"
 	md "github.com/go-gost/core/metadata"
 	xctx "github.com/go-gost/x/ctx"
+	xnet "github.com/go-gost/x/internal/net"
 	"github.com/go-gost/x/internal/net/proxyproto"
 	ssh_util "github.com/go-gost/x/internal/util/ssh"
 	"github.com/go-gost/x/registry"
@@ -70,6 +71,14 @@ func (d *sshDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialOp
 		if err != nil {
 			return
 		}
+		if d.md.tcpKeepalive {
+			xnet.ApplyKeepalive(conn, net.KeepAliveConfig{
+				Enable:   true,
+				Idle:     d.md.tcpKeepaliveIdle,
+				Interval: d.md.tcpKeepaliveInterval,
+				Count:    d.md.tcpKeepaliveCount,
+			})
+		}
 		if d.md.handshakeTimeout > 0 {
 			conn.SetDeadline(time.Now().Add(d.md.handshakeTimeout))
 			defer conn.SetDeadline(time.Time{})