fix: ensure execution stops after redirects

[sjinks]

What?

This PR ensures that code execution stops after calling wp_safe_redirect() in the two-factor authentication validation functions. Without exit(), PHP continues executing code after sending redirect headers, which could lead to security vulnerabilities or unintended behavior.

Why?

This follows WordPress best practices and aligns with the existing function documentation that states "wp_safe_redirect() does not exit automatically, and should almost always be followed by a call to exit;."

How?

Add exit() after calls to wp_safe_redirect().

Testing Instructions

N/A :-(

Screenshots or screencast

N/A

Changelog Entry

Security - Ensure that code execution stops after calling wp_safe_redirect().

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Co-authored-by: sjinks <volodymyrkolesnykov@git.wordpress.org>

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