Add mixed type hints to getArgument() method for PHP 8+ consistency

[zigzagdev]

Summary

This PR adds explicit mixed type hints to the getArgument() method in order to align the actual method signature with the existing PHPDoc annotations.

Motivation

• Clarifies function signature in accordance with PHP 8+
• Improves IDE support and static analysis
• Does not introduce any backward compatibility breaks

[zigzagdev]

Note: The CI failure seems unrelated to this PR.
The errors (e.g., dynamic ::class usage in Enqueue or deprecated test traits) are coming from unrelated packages or deprecated framework components that this PR does not touch.

Let me know if I should rebase or help isolate the failures in a separate PR.

[markstory]

In order to use mixed we would have to drop PHP 7.x support. I'd rather not do that in the 1.x branch as this branch supports CakePHP 4.x. However, this change could be made in the 2.x branch which has a min PHP version of 8.1.

[zigzagdev]

@markstory

Thank you for your reply!

After switching the base branch from 1.x to 2.x, I noticed that the changes I proposed have already been implemented there.
So I'll go ahead and close this PR. Thanks again for your time and feedback!