Template activation: call get_block_templates filter when short-circuiting

[ellatrix]

What?

Closes #71770

Why?

Some plugins expect get_block_templates to always be called, which no longer happens when we return a template through pre_get_block_templates.

How?

Run the get_block_templates filter ourselves before returning the new template.

Testing Instructions

add_filter(
	'get_block_templates',
	function ( $templates, $query ) {
		do_action( 'qm/debug', $query );
		array_map(
			function ( $template ) {
				do_action( 'qm/debug', $template ); // use any method to log data
			},
			$templates
		);
		return $templates;
	},
	10,
	2
);

Run this code to log when get_block_templates is called. For example, when activating a custom 404 template, it should still log a call with the 404 template, not just the index template.

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: ellatrix <[email protected]>
Co-authored-by: kmanijak <[email protected]>

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[kmanijak]

I confirm the hook is fired and so it's always called allowing plugins like WooCommerce to react on it and update templates. 🙌

[github-actions]

Flaky tests detected in 4ea6042.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/18170683104
📝 Reported issues:

• [Flaky Test] should refresh the page when stylesheet loading fails #71722 in /test/e2e/specs/interactivity/router-styles.spec.ts

[cbravobernal]

I just cherry-picked this PR to the release/21.8 branch to get it included in the next release: dc60d4e