refactor(handler/forward): extract 6 helpers from remote Handle, add 44 unit tests

Extract newRecorderObject, selectTarget, sniffingDial, buildSniffer,
handleSniffedProtocol, and handleRawForwarding from Handle (200→87 lines).
Move nil Router guard before sniffing for early failure. Log previously
silent sniff and pipe errors at Debug level. Apply readTimeout to Pipe.
Append :0 when target addr lacks a port. Reorder metadata fields and
add doc comments on all exported symbols.

handler/forward/remote/metadata.go

@@ -7,15 +7,15 @@ import (
 	"time"
 
 	"github.com/go-gost/core/bypass"
-	mdata "github.com/go-gost/core/metadata"
+	md "github.com/go-gost/core/metadata"
 	mdutil "github.com/go-gost/x/metadata/util"
 	"github.com/go-gost/x/registry"
 )
 
 type metadata struct {
 	readTimeout   time.Duration
-	proxyProtocol int
 	httpKeepalive bool
+	proxyProtocol int
 
 	sniffing                    bool
 	sniffingTimeout             time.Duration
@@ -28,11 +28,13 @@ type metadata struct {
 	mitmBypass  bypass.Bypass
 }
 
-func (h *forwardHandler) parseMetadata(md mdata.Metadata) (err error) {
+func (h *forwardHandler) parseMetadata(md md.Metadata) (err error) {
 	h.md.readTimeout = mdutil.GetDuration(md, "readTimeout")
 	if h.md.readTimeout <= 0 {
 		h.md.readTimeout = 15 * time.Second
 	}
+
+	h.md.httpKeepalive = mdutil.GetBool(md, "http.keepalive")
 	h.md.proxyProtocol = mdutil.GetInt(md, "proxyProtocol")
 
 	h.md.sniffing = mdutil.GetBool(md, "sniffing")
@@ -40,8 +42,6 @@ func (h *forwardHandler) parseMetadata(md mdata.Metadata) (err error) {
 	h.md.sniffingWebsocket = mdutil.GetBool(md, "sniffing.websocket")
 	h.md.sniffingWebsocketSampleRate = mdutil.GetFloat(md, "sniffing.websocket.sampleRate")
 
-	h.md.httpKeepalive = mdutil.GetBool(md, "http.keepalive")
-
 	certFile := mdutil.GetString(md, "mitm.certFile", "mitm.caCertFile")
 	keyFile := mdutil.GetString(md, "mitm.keyFile", "mitm.caKeyFile")
 	if certFile != "" && keyFile != "" {
@@ -57,5 +57,6 @@ func (h *forwardHandler) parseMetadata(md mdata.Metadata) (err error) {
 	}
 	h.md.alpn = mdutil.GetString(md, "mitm.alpn")
 	h.md.mitmBypass = registry.BypassRegistry().Get(mdutil.GetString(md, "mitm.bypass"))
+
 	return
 }