feat: 增强 starssh 的 agent forwarding 与 tcp/unix 转发能力

- 为 LoginInput 增加 ForwardSSHAgent 配置，并在 Exec/PTTY 会话创建时按需自动请求 agent forwarding
- 新增 agent_forward 运行时，封装本地 ssh-agent 建连、转发注册、显式请求与 unavailable/denied 语义
- 自动 agent forwarding 改为 best-effort：本地 agent 不可用、转发被拒绝或初始化失败时不再打断会话创建
- 为 StarSSH 增加 closing 状态与 agent forwarder 生命周期回收，避免 Close 与会话创建并发时泄漏资源
- 扩展 ForwardRequest 为带网络归一化的转发模型，支持 tcp/tcp4/tcp6/unix 端点组合
- 新增本地/远端 tcp<->unix、unix<->unix 及 detached helper，补齐 streamlocal 场景下的常用 API
- 将显式网络地址编码收口为 tcp4://、tcp6://、unix://，消除 tcp:22 一类值的解析歧义
- 为本地 unix listener 增加 stale socket 探测、复用与关闭清理，避免遗留 socket 导致重启失败
- 补充 agent forwarding、关闭竞态、remote unix forward、local unix forward、stale socket 复用与端点解析等回归测试

state.go

@@ -6,6 +6,8 @@ import (
 	"golang.org/x/crypto/ssh"
 )
 
+var errSSHClientClosing = errors.New("ssh client is closing")
+
 type sshClientRequester interface {
 	SendRequest(name string, wantReply bool, payload []byte) (bool, []byte, error)
 	Close() error
@@ -29,10 +31,19 @@ func (s *StarSSH) snapshotSSHClient() *ssh.Client {
 }
 
 func (s *StarSSH) requireSSHClient() (*ssh.Client, error) {
+	if s == nil {
+		return nil, errors.New("ssh client is nil")
+	}
+	if s.closing.Load() {
+		return nil, errSSHClientClosing
+	}
 	client := s.snapshotSSHClient()
 	if client == nil {
 		return nil, errors.New("ssh client is nil")
 	}
+	if s.closing.Load() {
+		return nil, errSSHClientClosing
+	}
 	return client, nil
 }
 
@@ -46,6 +57,7 @@ func (s *StarSSH) setTransport(client *ssh.Client, upstream *StarSSH) {
 	s.Client = client
 	s.upstream = upstream
 	s.online = client != nil
+	s.closing.Store(false)
 }
 
 func (s *StarSSH) detachTransport() (*ssh.Client, *StarSSH) {
@@ -84,7 +96,9 @@ func (s *StarSSH) closeTransport(waitKeepalive bool) error {
 		return nil
 	}
 
+	s.closing.Store(true)
 	_ = s.closeReusableSFTPClient()
+	agentForwarder := s.takeAgentForwarder()
 
 	client, upstream := s.detachTransport()
 	stop, done := s.takeKeepaliveHandles()
@@ -93,8 +107,13 @@ func (s *StarSSH) closeTransport(waitKeepalive bool) error {
 	}
 
 	var closeErr error
+	if agentForwarder != nil {
+		closeErr = normalizeAlreadyClosedError(agentForwarder.Close())
+	}
 	if client != nil {
-		closeErr = normalizeAlreadyClosedError(closeSSHClient(client))
+		if err := normalizeAlreadyClosedError(closeSSHClient(client)); closeErr == nil {
+			closeErr = err
+		}
 	}
 	if waitKeepalive && done != nil {
 		<-done
@@ -104,3 +123,13 @@ func (s *StarSSH) closeTransport(waitKeepalive bool) error {
 	}
 	return closeErr
 }
+
+func (s *StarSSH) canAttachAgentForwarder(client *ssh.Client) bool {
+	if s == nil || client == nil || s.closing.Load() {
+		return false
+	}
+
+	s.stateMu.RLock()
+	defer s.stateMu.RUnlock()
+	return !s.closing.Load() && s.Client == client
+}