Fix readme contributors validation for trailing commas

[ernilambar]

Issue

This is a pretty edge case. When contributors have a trailing comma, the code incorrectly triggers a readme_invalid_contributors error for valid contributors:

Minimal example:

Contributors: username1, username2, username3,

Meta handles this correctly. This PR brings parity with Meta.

This PR:

• Fixes the trim logic to properly handle trailing commas and spaces
• Adds a test case to verify contributors with trailing commas are handled correctly

[github-actions]

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Co-authored-by: ernilambar <nilambar@git.wordpress.org>
Co-authored-by: davidperezgar <davidperez@git.wordpress.org>
Co-authored-by: frantorres <frantorres@git.wordpress.org>

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