fix: 兼容 Claude Code v2.1.78+ 新 JSON 格式 metadata.user_id

Claude Code v2.1.78 起将 metadata.user_id 从拼接字符串改为 JSON：
旧: user_{hex}_account_{uuid}_session_{uuid}
新: {"device_id":"...","account_uuid":"...","session_id":"..."}

新增集中解析/格式化模块 metadata_userid.go：
- ParseMetadataUserID: 自动识别两种格式，提取 DeviceID/AccountUUID/SessionID
- FormatMetadataUserID: 根据 UA 版本输出对应格式（>= 2.1.78 输出 JSON）
- ExtractCLIVersion: 从 UA 提取版本号，消除与 ClaudeCodeValidator.ExtractVersion 的重复

修改消费者统一使用新模块：
- claude_code_validator: 用 ParseMetadataUserID 替代只匹配旧格式的 userIDPattern
- identity_service: RewriteUserID/WithMasking 增加 fingerprintUA 参数，
  解析用 ParseMetadataUserID，输出用 FormatMetadataUserID（版本感知）
- gateway_service: GenerateSessionHash 用 ParseMetadataUserID 提取 session_id，
  buildOAuthMetadataUserID 用 FormatMetadataUserID 输出版本匹配格式，
  两处 RewriteUserIDWithMasking 调用传入 fp.UserAgent
- account_test_service: generateSessionString 改用 FormatMetadataUserID，
  自动跟随 DefaultHeaders UA 版本

删除三个旧正则: userIDPattern, userIDRegex, sessionIDRegex
统一 hex 匹配为 [a-fA-F0-9]，修复旧 userIDRegex 只匹配小写的不一致

backend/internal/service/metadata_userid.go

@@ -0,0 +1,104 @@
+package service
+
+import (
+	"encoding/json"
+	"regexp"
+	"strings"
+)
+
+// NewMetadataFormatMinVersion is the minimum Claude Code version that uses
+// JSON-formatted metadata.user_id instead of the legacy concatenated string.
+const NewMetadataFormatMinVersion = "2.1.78"
+
+// ParsedUserID represents the components extracted from a metadata.user_id value.
+type ParsedUserID struct {
+	DeviceID    string // 64-char hex (or arbitrary client id)
+	AccountUUID string // may be empty
+	SessionID   string // UUID
+	IsNewFormat bool   // true if the original was JSON format
+}
+
+// legacyUserIDRegex matches the legacy user_id format:
+//
+//	user_{64hex}_account_{optional_uuid}_session_{uuid}
+var legacyUserIDRegex = regexp.MustCompile(`^user_([a-fA-F0-9]{64})_account_([a-fA-F0-9-]*)_session_([a-fA-F0-9-]{36})$`)
+
+// jsonUserID is the JSON structure for the new metadata.user_id format.
+type jsonUserID struct {
+	DeviceID    string `json:"device_id"`
+	AccountUUID string `json:"account_uuid"`
+	SessionID   string `json:"session_id"`
+}
+
+// ParseMetadataUserID parses a metadata.user_id string in either format.
+// Returns nil if the input cannot be parsed.
+func ParseMetadataUserID(raw string) *ParsedUserID {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return nil
+	}
+
+	// Try JSON format first (starts with '{')
+	if raw[0] == '{' {
+		var j jsonUserID
+		if err := json.Unmarshal([]byte(raw), &j); err != nil {
+			return nil
+		}
+		if j.DeviceID == "" || j.SessionID == "" {
+			return nil
+		}
+		return &ParsedUserID{
+			DeviceID:    j.DeviceID,
+			AccountUUID: j.AccountUUID,
+			SessionID:   j.SessionID,
+			IsNewFormat: true,
+		}
+	}
+
+	// Try legacy format
+	matches := legacyUserIDRegex.FindStringSubmatch(raw)
+	if matches == nil {
+		return nil
+	}
+	return &ParsedUserID{
+		DeviceID:    matches[1],
+		AccountUUID: matches[2],
+		SessionID:   matches[3],
+		IsNewFormat: false,
+	}
+}
+
+// FormatMetadataUserID builds a metadata.user_id string in the format
+// appropriate for the given CLI version. Components are the rewritten values
+// (not necessarily the originals).
+func FormatMetadataUserID(deviceID, accountUUID, sessionID, uaVersion string) string {
+	if IsNewMetadataFormatVersion(uaVersion) {
+		b, _ := json.Marshal(jsonUserID{
+			DeviceID:    deviceID,
+			AccountUUID: accountUUID,
+			SessionID:   sessionID,
+		})
+		return string(b)
+	}
+	// Legacy format
+	return "user_" + deviceID + "_account_" + accountUUID + "_session_" + sessionID
+}
+
+// IsNewMetadataFormatVersion returns true if the given CLI version uses the
+// new JSON metadata.user_id format (>= 2.1.78).
+func IsNewMetadataFormatVersion(version string) bool {
+	if version == "" {
+		return false
+	}
+	return CompareVersions(version, NewMetadataFormatMinVersion) >= 0
+}
+
+// ExtractCLIVersion extracts the Claude Code version from a User-Agent string.
+// Returns "" if the UA doesn't match the expected pattern.
+func ExtractCLIVersion(ua string) string {
+	matches := claudeCodeUAVersionPattern.FindStringSubmatch(ua)
+	if len(matches) >= 2 {
+		return matches[1]
+	}
+	return ""
+}