refactor: 重构 starssh 核心运行时并补强 ssh/exec/terminal/sftp 能力

- 拆分原有单体 ssh.go，按职责重组为 types、utils、transport、login、keepalive、session、exec、pool、shell、terminal、forward、hostkey、state 等模块，并补充平台相关实现
  - 重做登录与连接运行时，补齐基于 context 的建连、jump/proxy 链路、可配置认证顺序，以及 Unix/Windows 下的 ssh-agent 支持
  - 新增正式非交互执行模型 ExecRequest/ExecResult，支持流式输出、溢出统计、超时控制，以及 posix/powershell/cmd/raw 多方言执行
  - 保留旧 shell 风格兼容接口，同时让路径/用户探测等 helper 具备跨 shell fallback，避免 Windows 目标继续硬依赖 POSIX 命令
  - 新增 TerminalSession 作为原始交互终端基座，提供 IO attach、resize、signal/control、退出状态与关闭原因管理
  - 重构端口转发语义，默认复用当前 SSH 连接，并显式提供 detached 的本地/动态转发模式承载隔离场景
  - 梳理 keepalive 与取消语义，区分仅取消本次操作和关闭整条连接，并统一连接状态与传输关闭路径
  - 围绕新的 session/连接生命周期重做执行池与运行时支撑
  - 大幅增强 SFTP 传输链路，补齐更安全的原子替换、校验、进度回调、重试隔离、可复用 client 生命周期与失败语义
  - 新增取消语义、keepalive、SFTP、forward、terminal input 等关键回归测试，提升核心链路稳定性

utils.go

@@ -0,0 +1,162 @@
+package starssh
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+	"time"
+)
+
+var (
+	ansiCSIRegexp    = regexp.MustCompile(`\x1b\[[0-9;?]*[ -/]*[@-~]`)
+	ansiOSCRegexp    = regexp.MustCompile(`\x1b\][^\x07]*(\x07|\x1b\\)`)
+	leadingIntRegexp = regexp.MustCompile(`^[+-]?\d+`)
+)
+
+func SedColor(str string) string {
+	return stripControlSequences(str)
+}
+
+func normalizeShellOutput(raw string) string {
+	return strings.TrimSpace(strings.ReplaceAll(raw, "\r\n", "\n"))
+}
+
+func stripControlSequences(raw string) string {
+	cleaned := ansiOSCRegexp.ReplaceAllString(raw, "")
+	cleaned = ansiCSIRegexp.ReplaceAllString(cleaned, "")
+	cleaned = strings.ReplaceAll(cleaned, "\r", "")
+	cleaned = strings.Map(func(r rune) rune {
+		if r == '\n' || r == '\t' {
+			return r
+		}
+		if r < 0x20 || r == 0x7f {
+			return -1
+		}
+		return r
+	}, cleaned)
+	return cleaned
+}
+
+func stripLeadingEcho(output string, command string, markerCommand string) string {
+	result := output
+	if command == "" {
+		return result
+	}
+
+	withMarker := command
+	if markerCommand != "" {
+		withMarker += "\n" + markerCommand
+	}
+	if strings.HasPrefix(result, withMarker) {
+		return strings.TrimSpace(strings.TrimPrefix(result, withMarker))
+	}
+	if strings.HasPrefix(result, command) {
+		return strings.TrimSpace(strings.TrimPrefix(result, command))
+	}
+	return result
+}
+
+func collectLinesWithoutTokens(output string, tokens ...string) string {
+	lines := strings.Split(output, "\n")
+	filtered := make([]string, 0, len(lines))
+
+	for _, line := range lines {
+		skip := false
+		for _, token := range tokens {
+			if token != "" && strings.Contains(line, token) {
+				skip = true
+				break
+			}
+		}
+		if !skip {
+			filtered = append(filtered, line)
+		}
+	}
+
+	return strings.TrimSpace(strings.Join(filtered, "\n"))
+}
+
+func collectLinesForCommandOutput(output string, promptToken string, tokens ...string) string {
+	lines := strings.Split(output, "\n")
+	filtered := make([]string, 0, len(lines))
+
+	for _, line := range lines {
+		skip := false
+		for _, token := range tokens {
+			if token != "" && strings.Contains(line, token) {
+				skip = true
+				break
+			}
+		}
+		if !skip && promptToken != "" && strings.Contains(line, promptToken) {
+			skip = true
+		}
+		if !skip {
+			filtered = append(filtered, line)
+		}
+	}
+
+	return strings.TrimSpace(strings.Join(filtered, "\n"))
+}
+
+func shellSingleQuote(s string) string {
+	return "'" + strings.ReplaceAll(s, "'", "'\"'\"'") + "'"
+}
+
+func normalizeBufferSize(bufcap int) int {
+	if bufcap <= 0 {
+		return defaultTransferBufferSize
+	}
+	return bufcap
+}
+
+func newCommandTokens() (beginToken string, endToken string) {
+	nonce := newNonce(8)
+	return "__STARSSH_BEGIN_" + nonce + "__", "__STARSSH_END_" + nonce + "__"
+}
+
+func newNonce(size int) string {
+	if size <= 0 {
+		size = 8
+	}
+
+	buf := make([]byte, size)
+	if _, err := rand.Read(buf); err != nil {
+		return fmt.Sprintf("%d", time.Now().UnixNano())
+	}
+	return strings.ToUpper(hex.EncodeToString(buf))
+}
+
+func splitByEndToken(output string, endToken string) (before string, exitCode int, found bool, parseErr error) {
+	prefix := endToken + ":"
+	lines := strings.Split(output, "\n")
+	beforeLines := make([]string, 0, len(lines))
+
+	for _, line := range lines {
+		trimmedLine := strings.TrimSpace(line)
+		if !strings.HasPrefix(trimmedLine, prefix) {
+			beforeLines = append(beforeLines, line)
+			continue
+		}
+
+		codeText := strings.TrimSpace(trimmedLine[len(prefix):])
+		match := leadingIntRegexp.FindString(codeText)
+		if match == "" || match != codeText {
+			beforeLines = append(beforeLines, line)
+			continue
+		}
+
+		code, err := strconv.Atoi(match)
+		if err != nil {
+			beforeLines = append(beforeLines, line)
+			continue
+		}
+
+		return strings.Join(beforeLines, "\n"), code, true, nil
+	}
+
+	return strings.Join(beforeLines, "\n"), 0, false, nil
+}