feat: add PHP_CodeSniffer 4.x support

[MickaelN]

Summary

Upgrades squizlabs/php_codesniffer from ^3.13.5 to ^4.0 and slevomat/coding-standard from ^8.22.1 to ^8.23.

Closes #723

Changes

Core:

• File.php: typed addMessage() signature, fixableCount replaced by fixableErrorCount/fixableWarningCount
• SniffDecorator.php: typed process() parameter
• ForbiddenSetterSniff.php: typed process() parameter, dotted error code to comply with PHPCS 4 naming validation, getDeclarationName empty string check

Removed sniffs (dropped in PHPCS 4):

• Zend\Debug\CodeAnalyzerSniff — removed without replacement (redundant with SyntaxCheck which uses parallel-lint)
• Generic\Functions\CallTimePassByReferenceSniff — removed without replacement (call-time pass-by-reference is a syntax error since PHP 5.4, irrelevant for PHP 8.3+)

Moved sniff:

• Squiz\WhiteSpace\LanguageConstructSpacingSniff → Generic\WhiteSpace\LanguageConstructSpacingSniff

Tests:

• TestCase::prepareFixtureWithSniff(): manual sniff registration to bypass PHPCS 4 namespace convention validation in populateTokenListeners() for custom sniffs outside the Standard\Sniffs\Category namespace

Verification

• 100 tests pass (0 failures, 0 errors)
• Self-analysis produces scores
• Tested on a large Symfony project (~1900 files): scores are consistent with PHPCS 3 baseline (Code, Complexity and Architecture identical, Style within 1.2 pts)