refactor(core): centralize reserved system user identity

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

lib/Providers/Provider.php

@@ -14,6 +14,7 @@ use KTXC\Resource\ProviderManager;
 use KTXF\Mail\Provider\ProviderBaseInterface;
 use KTXF\Mail\Provider\ProviderServiceMutateInterface;
 use KTXF\Resource\Provider\ResourceServiceMutateInterface;
+use KTXF\Resource\SystemIdentity;
 use KTXM\ProviderMailSystem\Stores\ServiceStore;
 
 /**
@@ -108,7 +109,7 @@ class Provider implements ProviderBaseInterface, ProviderServiceMutateInterface
     {
         // routes are tenant scoped and only visible in the system user context,
         // keeping them out of regular user account listings
-        if ($userId !== self::USER_SYSTEM) {
+        if ($userId !== SystemIdentity::USER) {
             return [];
         }
 
@@ -122,7 +123,7 @@ class Provider implements ProviderBaseInterface, ProviderServiceMutateInterface
 
     public function serviceFetch(string $tenantId, string $userId, string|int $identifier): ?Service
     {
-        if ($userId !== self::USER_SYSTEM) {
+        if ($userId !== SystemIdentity::USER) {
             return null;
         }
 
@@ -147,7 +148,7 @@ class Provider implements ProviderBaseInterface, ProviderServiceMutateInterface
         if (!str_ends_with($address, self::ADDRESS_DOMAIN_SUFFIX)) {
             return null;
         }
-        if ($userId !== self::USER_SYSTEM) {
+        if ($userId !== SystemIdentity::USER) {
             return null;
         }
 
@@ -170,7 +171,7 @@ class Provider implements ProviderBaseInterface, ProviderServiceMutateInterface
 
     public function serviceExtant(string $tenantId, string $userId, string|int ...$identifiers): array
     {
-        if ($userId !== self::USER_SYSTEM) {
+        if ($userId !== SystemIdentity::USER) {
             return array_fill_keys(array_map('strval', $identifiers), false);
         }
 
@@ -223,7 +224,7 @@ class Provider implements ProviderBaseInterface, ProviderServiceMutateInterface
      */
     protected function assertSystemContext(string $userId): void
     {
-        if ($userId !== self::USER_SYSTEM) {
+        if ($userId !== SystemIdentity::USER) {
             throw new InvalidArgumentException('System mail routes can only be managed in the system user context');
         }
     }