test: add service layer unit tests for webhook/metadata/error/config

- webhook_service_test.go: isPrivateIP, isSafeURL, computeHMAC
- request_metadata_test.go: context functions
- classified_error_test.go: error types
- config_defaults_test.go: password reset/SMS defaults
- email_config_test.go: email code defaults
- auth_runtime_test.go: isUserNotFoundError

Service coverage: 11.2% -> 14.7%

internal/service/auth_runtime_test.go

@@ -0,0 +1,75 @@
+package service
+
+import (
+	"errors"
+	"testing"
+
+	"gorm.io/gorm"
+)
+
+// =============================================================================
+// Auth Runtime Helper Functions Tests
+// =============================================================================
+
+func TestIsUserNotFoundError(t *testing.T) {
+	tests := []struct {
+		name     string
+		err      error
+		expected bool
+	}{
+		{
+			name:     "nil error",
+			err:      nil,
+			expected: false,
+		},
+		{
+			name:     "gorm record not found",
+			err:      gorm.ErrRecordNotFound,
+			expected: true,
+		},
+		{
+			name:     "wrapped gorm record not found",
+			err:      errors.Join(gorm.ErrRecordNotFound, errors.New("additional context")),
+			expected: true,
+		},
+		{
+			name:     "other error",
+			err:      errors.New("some other error"),
+			expected: false,
+		},
+		{
+			name:     "generic error",
+			err:      errors.New("something went wrong"),
+			expected: false,
+		},
+		{
+			name:     "error containing user not found",
+			err:      errors.New("user not found"),
+			expected: true, // contains "user not found" in lowercase
+		},
+		{
+			name:     "error containing record not found",
+			err:      errors.New("record not found"),
+			expected: true, // contains "record not found"
+		},
+		{
+			name:     "error containing not found",
+			err:      errors.New("entity not found"),
+			expected: true, // contains "not found"
+		},
+		{
+			name:     "error containing 用户不存在",
+			err:      errors.New("用户不存在"),
+			expected: true, // contains Chinese "用户不存在"
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := isUserNotFoundError(tt.err)
+			if result != tt.expected {
+				t.Errorf("isUserNotFoundError(%v) = %v, want %v", tt.err, result, tt.expected)
+			}
+		})
+	}
+}