[sarequl]

Fixes a fatal "Undefined array key 0" in the editor when a plugin-registered block template is included in the current template list.

See https://core.trac.wordpress.org/ticket/64216

What

get_block_templates() merges plugin-registered templates into the query results with array_merge(). WP_Block_Templates_Registry::get_by_query() keys its result by template name, so array_filter()/array_map() preserve those string keys and the merged array is no longer a list. wp-includes/block-editor.php then reads $current_template[0]->content, which is undefined, so the editor dies with "Undefined array key 0" when a plugin-registered template is among the matches.

Why

Plugin-registered templates are a supported extension point, and selecting one as the current template must not fatal. The reporter's diagnosis and suggested fix — wrap the registered templates in array_values() — is confirmed in code: the registry keys results by template name, so reindexing before the merge restores the list shape the editor expects.

How

In src/wp-includes/block-template-utils.php, array_merge( $query_result, $matching_registered_templates ) becomes array_merge( $query_result, array_values( $matching_registered_templates ) ). This reindexes the registered templates so the merged result is a list, matching the rest of the template API. No other behaviour changes.

Adds test_get_block_templates_should_return_a_list_when_a_plugin_registered_template_is_included in tests/phpunit/tests/blocks/getBlockTemplates.php with @ticket 64216, asserting array_is_list(), assertCount( 1, ... ) and $templates[0]->slug.

Testing Instructions

1. Run the tests: vendor/bin/phpunit tests/phpunit/tests/blocks/getBlockTemplates.php → OK (8 tests, 26 assertions).
2. Negative control: revert the one source line (git checkout <base> -- src/wp-includes/block-template-utils.php) and re-run → Tests: 8, Assertions: 24, Failures: 1, failing exactly test_get_block_templates_should_return_a_list_when_a_plugin_registered_template_is_included. Restoring the fix makes the run green again.
3. Broader regression: vendor/bin/phpunit --test-suffix .php tests/phpunit/tests/blocks/ → OK (576 tests, 3151 assertions).
4. phpcs on the two changed files: vendor/bin/phpcs --standard=phpcs.xml.dist src/wp-includes/block-template-utils.php tests/phpunit/tests/blocks/getBlockTemplates.php → exit 0.
5. composer lint and composer phpstan show only pre-existing violations/errors (identical on base and branch); npm run build passes.

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