feat: use callable instead of Closure|array|string for handler type

[CodeWithKyrian]

This PR updates handler type hints from \Closure|array|string to callable|array|string throughout the codebase.

Changes

• Updated all RegisteredElement classes to use callable type hint
• Fixed RegisteredElement constructor to properly handle callable parameter
• Updated ServerBuilder and Registry method signatures

Why

The callable type hint is more semantically correct and covers the same use cases as \Closure|array|string but is cleaner and more inclusive.

Breaking Changes

None - this is a type hint improvement that maintains backward compatibility.