Bump phpunit/phpunit from 10.5.62 to 12.5.8

- Update schema URL in phpunit.xml.dist to 12.0
- Remove deprecated E_STRICT constant from bootstrap.php
- Modernize ClientTest: typed properties, createStub(), ::class syntax

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kornrunner

composer.json

@@ -22,7 +22,7 @@
 		"symfony/console" : "~5 || ~6 || ~7"
 	},
 	"require-dev" : {
-		"phpunit/phpunit" : "^10.0"
+		"phpunit/phpunit" : "^12.0"
 	},
 	"autoload" : {
 		"psr-4" : {