Update PHPStan to version 2.x for improved type analysis

[Copilot]

This PR updates PHPStan from version 1.x to 2.x to take advantage of improved type inference and analysis capabilities.

Changes Made

Dependency Update:

• Updated phpstan/phpstan constraint from ^1.11 to ^2.0 in composer.json
• Upgraded from PHPStan 1.12.29 to 2.1.28 (latest stable)

Compatibility Fixes:

• Fixed PHPDoc annotations in src/Option/Option.php to remove incorrect generic type references for the None class
• Updated PHPStan configuration to ignore unused testing traits (expected behavior for utility traits)
• Updated type assertions in test files to match PHPStan 2's more precise type inference:
  • strlen() now correctly returns int<0, max> instead of generic int
  • List collections now use list<T> instead of array<int, T>
  • String type casting uses more specific string intersection types

Benefits:

• More accurate type analysis with precise integer ranges and string types
• Better performance and future-proofing
• Enhanced static analysis capabilities

All tests continue to pass and the GitHub Actions workflow automatically uses the new PHPStan version without requiring workflow changes.

Fixes #6.

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[erikgaal]

Well done!