Improve code quality: fix bugs, add type safety, and harden tests

[JayBizzle]

• Fix null concatenation deprecation in setUserAgent (PHP 8.1+)
• Fix MSIE exclusion pattern missing * quantifier
• Reset matches at top of isCrawler() for reliable state
• Add preg error handling in isCrawler()
• Make compileRegex() protected (internal API)
• Add return type declarations to all methods
• Add parameter type hints throughout
• Use setUp() in tests, fix property visibility/naming
• Fix typo in test method name (persit -> persist)
• Optimize collision test from O(n²) to O(n²/2)
• Add tests for regex validity and stored agent immutability