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/ws/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"
 	ws_util "github.com/go-gost/x/internal/util/ws"
 	"github.com/go-gost/x/registry"
@@ -64,6 +65,15 @@ func (d *wsDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialOpt
 		d.options.Logger.Error(err)
 	}
 
+	if d.md.tcpKeepalive {
+		xnet.ApplyKeepalive(conn, net.KeepAliveConfig{
+			Enable:   true,
+			Idle:     d.md.tcpKeepaliveIdle,
+			Interval: d.md.tcpKeepaliveInterval,
+			Count:    d.md.tcpKeepaliveCount,
+		})
+	}
+
 	conn = proxyproto.WrapClientConn(
 		d.options.ProxyProtocol,
 		xctx.SrcAddrFromContext(ctx),

dialer/ws/metadata.go

@@ -25,6 +25,11 @@ type metadata struct {
 
 	header            http.Header
 	keepaliveInterval time.Duration
+
+	tcpKeepalive         bool
+	tcpKeepaliveIdle     time.Duration
+	tcpKeepaliveInterval time.Duration
+	tcpKeepaliveCount    int
 }
 
 func (d *wsDialer) parseMetadata(md mdata.Metadata) (err error) {
@@ -56,5 +61,10 @@ func (d *wsDialer) parseMetadata(md mdata.Metadata) (err error) {
 		}
 	}
 
+	d.md.tcpKeepalive = mdutil.GetBool(md, "tcp.keepalive")
+	d.md.tcpKeepaliveIdle = mdutil.GetDuration(md, "tcp.keepalive.idle")
+	d.md.tcpKeepaliveInterval = mdutil.GetDuration(md, "tcp.keepalive.interval")
+	d.md.tcpKeepaliveCount = mdutil.GetInt(md, "tcp.keepalive.count")
+
 	return
 }