Add validation for '@' character in custom option keys per OData specification

Co-authored-by: anderly <573151+anderly@users.noreply.github.com>

Author: Copilot

src/Query/Builder.php

@@ -269,7 +269,7 @@ public function whereKey($id)
      * Array format: ['key' => 'value', 'key2' => 'value2']
      * 
      * Custom option keys must follow OData naming conventions:
-     * - Must not start with '$' (reserved for standard OData parameters)
+     * - Must not start with '$' or '@' (reserved for standard OData parameters)
      * - Must be valid identifiers (alphanumeric and underscores)
      * - Cannot be empty
      * 
@@ -370,9 +370,9 @@ protected function validateCustomOptionKey($key)
             throw new \InvalidArgumentException('Custom option key must be a non-empty string');
         }
 
-        // Check if key starts with '$' (reserved for OData system parameters)
-        if (strpos($key, '$') === 0) {
-            throw new \InvalidArgumentException("Custom option key '$key' cannot start with '\$' (reserved for OData system parameters)");
+        // Check if key starts with '$' or '@' (reserved for OData system parameters)
+        if (strpos($key, '$') === 0 || strpos($key, '@') === 0) {
+            throw new \InvalidArgumentException("Custom option key '$key' cannot start with '\$' or '@' (reserved for OData system parameters)");
         }
 
         // Check for valid identifier pattern (alphanumeric, underscores, hyphens)

tests/Query/CustomOptionsTest.php

@@ -170,12 +170,21 @@ public function testAddOptionIgnoresNullAndEmpty()
     public function testAddOptionValidatesKeyStartingWithDollar()
     {
         $this->expectException(\InvalidArgumentException::class);
-        $this->expectExceptionMessage('Custom option key \'$invalid\' cannot start with \'$\'');
+        $this->expectExceptionMessage('Custom option key \'$invalid\' cannot start with \'$\' or \'@\'');
 
         $builder = $this->getBuilder();
         $builder->addOption(['$invalid' => 'value']);
     }
 
+    public function testAddOptionValidatesKeyStartingWithAt()
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('Custom option key \'@invalid\' cannot start with \'$\' or \'@\'');
+        
+        $builder = $this->getBuilder();
+        $builder->addOption(['@invalid' => 'value']);
+    }
+
     public function testAddOptionValidatesEmptyKey()
     {
         $this->expectException(\InvalidArgumentException::class);