fix: fix phpstan errors

[Maxcastel]

Fix the followig errors:

PHPStan - PHP Static Analysis Tool 2.1.50
Note: Using configuration file /home/runner/work/core/core/phpstan.neon.dist.
Error: Property Mcp\Schema\Request\CallToolRequest::$arguments (array<string, mixed>) on left side of ?? is not nullable.
Error: Property Doctrine\ORM\Mapping\AssociationMapping::$targetEntity (class-string) on left side of ?? is not nullable.
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  Line   src/Mcp/Server/Handler.php                                             
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  77     Property Mcp\Schema\Request\CallToolRequest::$arguments (array<string  
         , mixed>) on left side of ?? is not nullable.                          
         🪪  nullCoalesce.property                                              
 ------ ----------------------------------------------------------------------- 

 ------ ----------------------------------------------------------------- 
  Line   src/Symfony/Doctrine/EventListener/PurgeHttpCacheListener.php    
 ------ ----------------------------------------------------------------- 
  148    Property Doctrine\ORM\Mapping\AssociationMapping::$targetEntity  
         (class-string) on left side of ?? is not nullable.               
         🪪  nullCoalesce.property                                        
 ------ -----------------------------------------------------------------