Add validation to prevent feed names starting with an underscore.

[snehapatil2001]

Ticket: https://core.trac.wordpress.org/ticket/59945

• This PR addresses an issue in the add_feed function where feed names starting with an underscore are not handled properly, leading to errors.
• The function now includes validation to prevent feed names from starting with an underscore, improving overall robustness and developer experience.

Background:

• The add_feed function allows developers to register custom feed types in WordPress. However, if the feed name starts with an underscore, it is stripped by the do_feed function, causing unexpected errors.
• This PR introduces a validation step to ensure feed names do not start with an underscore, thereby avoiding these issues.

Changes Made:

Validation in add_feed Function:

• Added a check to ensure the feed name does not start with an underscore (_).
• If the feed name is invalid, an error is logged using error_log, and the function returns false.

Proper Initialization Check:

• Ensured that the $wp_rewrite global variable is initialized before proceeding with feed registration.

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[github-actions]

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