Code Quality: Resolve the isset.variable PHPStan errors.
Each of the six baselined errors was an isset() on a variable that is always defined and not nullable, or never defined at all. The $_POST superglobal is always set, so the checks on it in Custom_Image_Header::step_2() and get_media_item() were unconditional; the latter becomes ! empty( $_POST ), which is what isset() plus count() already meant. PHP_VERSION_ID is constant within a request, so the guard above the $loader check in WP_oEmbed::_parse_xml() already decides it. The namespace group in WP_Block_Parser::next_token() is a non-trailing optional group under PREG_OFFSET_CAPTURE, which PHP always populates. In wp_edit_theme_plugin_file() the only assignment to $stylesheet is guarded by a ! empty() on the argument it comes from, so a truthiness test cannot diverge from isset().
The remaining error is the reverse case: $s in load_template() is undefined to static analysis because it arrives through extract(). Assigning the query vars to a local first, and annotating that local as array{ s?: scalar, ... }, resolves both that report and the inline @phpstan-ignore above it. The annotation is deliberately local rather than on WP_Query::$query_vars, where an unsealed shape makes every unnamed key "might not exist" and reports more than it resolves. scalar rather than string is what WP_Query::parse_query() actually guarantees, since it gates the value only with is_scalar().
With the last entry fixed, the baseline file and its entry in the includes of phpstan.neon.dist are removed.
Developed in https://github.com/WordPress/wordpress-develop/pull/13023.
Follow-up to r28407, r32298, r41721, r48789, r60351, r61504, r61699, r63019.
Props westonruter, apermo.
See #65817.