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/generate_session_hash_test.go

@@ -24,7 +24,7 @@ func TestGenerateSessionHash_MetadataHasHighestPriority(t *testing.T) {
 	svc := &GatewayService{}
 
 	parsed := &ParsedRequest{
-		MetadataUserID: "session_123e4567-e89b-12d3-a456-426614174000",
+		MetadataUserID: "user_a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2_account__session_123e4567-e89b-12d3-a456-426614174000",
 		System:         "You are a helpful assistant.",
 		HasSystem:      true,
 		Messages: []any{
@@ -196,7 +196,7 @@ func TestGenerateSessionHash_MetadataOverridesSessionContext(t *testing.T) {
 	svc := &GatewayService{}
 
 	parsed := &ParsedRequest{
-		MetadataUserID: "session_123e4567-e89b-12d3-a456-426614174000",
+		MetadataUserID: "user_a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2_account__session_123e4567-e89b-12d3-a456-426614174000",
 		Messages: []any{
 			map[string]any{"role": "user", "content": "hello"},
 		},
@@ -212,6 +212,22 @@ func TestGenerateSessionHash_MetadataOverridesSessionContext(t *testing.T) {
 		"metadata session_id should take priority over SessionContext")
 }
 
+func TestGenerateSessionHash_MetadataJSON_HasHighestPriority(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed := &ParsedRequest{
+		MetadataUserID: `{"device_id":"a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2","account_uuid":"","session_id":"c72554f2-1234-5678-abcd-123456789abc"}`,
+		System:         "You are a helpful assistant.",
+		HasSystem:      true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+	}
+
+	hash := svc.GenerateSessionHash(parsed)
+	require.Equal(t, "c72554f2-1234-5678-abcd-123456789abc", hash, "JSON format metadata session_id should have highest priority")
+}
+
 func TestGenerateSessionHash_NilSessionContextBackwardCompatible(t *testing.T) {
 	svc := &GatewayService{}