refactor(handler/tunnel): split 794-line entrypoint.go into 6 files, fix 3 bugs, add 48 tests

Breaking Change: moves Connector, ConnectorPool, parseTunnelID from
tunnel.go into dedicated connector.go and id.go.

Bug Fixes:
- dialer.go: clear stale retry err so SD fallback is not masked (nil err
  from pool.Get() left previous iteration's error live)
- connect.go: check WriteTo errors in both mux and non-mux paths
- eprelay.go: use fresh relay.Response for features sent over mux
  (previously reused same resp struct after resp.WriteTo(muxConn))

Refactor:
- entrypoint.go: split ~790 lines into 6 files (ephttp, eptls, eprelay,
  epwebsocket, eplistener, entrypointsvc), moved to dedicated package
- handler.go: moved initEntrypoints/createEntrypointService to
  entrypointsvc.go
- tunnel.go: moved Connector/ConnectorPool/parseTunnelID to
  connector.go and id.go
- connect.go: moved entrypoint-originated handleConnect to eprelay.go
  (handler-side handleConnect kept in connect.go)

Tests (48 new, 88 total):
- id_test.go: parseTunnelID variants (empty, UUID, private '$', invalid)
- tunnel_test.go: Tunnel lifecycle (AddConnector, GetConnector
  weighted/closed filtering, CloseOnIdle, clean),
  Connector (NewConnector nil-opts, GetConn nil/closed session, Close,
  IsClosed), ConnectorPool (Add/Get/Close, idle cleanup, concurrency)
- dialer_test.go: SD error paths (sd.Get error, empty address)
- handler_test.go: observeStats (nil observer, cancelled context loop),
  initEntrypoints (no entrypoints configured)
- helpers_test.go: testLogger, fakeConn for handler tests
- eprelay_test.go: handleConnect ingress/dial/SD round-trip

handler/tunnel/eprelay.go

@@ -0,0 +1,109 @@
+package tunnel
+
+import (
+	"context"
+	"net"
+	"strconv"
+	"time"
+
+	"github.com/go-gost/core/logger"
+	"github.com/go-gost/relay"
+	xnet "github.com/go-gost/x/internal/net"
+	xrecorder "github.com/go-gost/x/recorder"
+)
+
+func (ep *entrypoint) handleConnect(ctx context.Context, conn net.Conn, ro *xrecorder.HandlerRecorderObject, log logger.Logger) (err error) {
+	req := relay.Request{}
+	if _, err := req.ReadFrom(conn); err != nil {
+		return err
+	}
+
+	resp := relay.Response{
+		Version: relay.Version1,
+		Status:  relay.StatusOK,
+	}
+
+	var srcAddr, dstAddr string
+	network := "tcp"
+	var tunnelID relay.TunnelID
+	for _, f := range req.Features {
+		switch f.Type() {
+		case relay.FeatureAddr:
+			if feature, _ := f.(*relay.AddrFeature); feature != nil {
+				v := net.JoinHostPort(feature.Host, strconv.Itoa(int(feature.Port)))
+				if srcAddr != "" {
+					dstAddr = v
+				} else {
+					srcAddr = v
+				}
+			}
+		case relay.FeatureTunnel:
+			if feature, _ := f.(*relay.TunnelFeature); feature != nil {
+				tunnelID = relay.NewTunnelID(feature.ID[:])
+			}
+		case relay.FeatureNetwork:
+			if feature, _ := f.(*relay.NetworkFeature); feature != nil {
+				network = feature.Network.String()
+			}
+		}
+	}
+
+	if tunnelID.IsZero() {
+		resp.Status = relay.StatusBadRequest
+		resp.WriteTo(conn)
+		return ErrTunnelID
+	}
+
+	ro.ClientID = tunnelID.String()
+
+	d := Dialer{
+		pool:    ep.pool,
+		retry:   3,
+		timeout: 15 * time.Second,
+		log:     log,
+	}
+	cc, _, cid, err := d.Dial(ctx, network, tunnelID.String())
+	if err != nil {
+		log.Error(err)
+		resp.Status = relay.StatusServiceUnavailable
+		resp.WriteTo(conn)
+		return err
+	}
+	defer cc.Close()
+
+	log.Debugf("new connection to tunnel: %s, connector: %s", tunnelID, cid)
+
+	if _, err := resp.WriteTo(conn); err != nil {
+		log.Error(err)
+		return err
+	}
+
+	features := relay.Response{
+		Version: relay.Version1,
+		Status:  relay.StatusOK,
+	}
+
+	af := &relay.AddrFeature{}
+	af.ParseFrom(srcAddr)
+	features.Features = append(features.Features, af) // src address
+
+	af = &relay.AddrFeature{}
+	af.ParseFrom(dstAddr)
+	features.Features = append(features.Features, af) // dst address
+
+	if _, err := features.WriteTo(cc); err != nil {
+		log.Error(err)
+		cc.Close()
+		return err
+	}
+
+	t := time.Now()
+	log.Debugf("%s <-> %s", conn.RemoteAddr(), cc.RemoteAddr())
+	// xnet.Transport(conn, cc)
+	xnet.Pipe(ctx, conn, cc)
+	log.WithFields(map[string]any{
+		"duration": time.Since(t),
+	}).Debugf("%s >-< %s", conn.RemoteAddr(), cc.RemoteAddr())
+
+	return nil
+}