Add unit test for get_post_galleries() when parsing malformed [gallery] shortcodes.

[rollybueno]

Summary

This PR adds a PHPUnit test that reproduces the issue reported in Trac #63577, where a warning is triggered when get_post_galleries() encounters malformed [gallery] shortcodes.

Specifically, the issue occurs when the gallery array lacks the expected 'id' key, and functions like wp_list_pluck() assume it exists. This test demonstrates how [gallery], [gallery ids], or [gallery ids=""] can result in a malformed output that reproduces the warning.

Testing Instructions

Run the test class:

npm run test:php -- --filter getPostGalleries

Trac ticket: https://core.trac.wordpress.org/ticket/63577

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