Fix template modified return value for file templates

[ntsekouras]

Trac ticket: https://core.trac.wordpress.org/ticket/65730

Backports: WordPress/gutenberg#80733

What?

Recently we added the date field for template and template parts (https://core.trac.wordpress.org/ticket/65049) and it was noted that the returned value from REST API could return false for file templates. This will be fixed here: https://core.trac.wordpress.org/ticket/65728 for 7.1.

While looking into the above issue, I observed that we have the same issue for modified property since 6.3.

We should update the type to be the same with date to string|null and not return false for file templates.

This PR updates the modified prop to also return null for file templates.

Testing instructions

1. Log in and ensure you have a block theme enabled
2. Test the templates REST endpoint in dev tools: const templates = await wp.apiFetch( { path: '/wp/v2/templates?per_page=100' } );. Then inspect templates
3. Observe that modified is null for file templates.

Use of AI Tools

Opus 5 with direction, changes and review

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Pull request overview

This PR updates the Templates REST API to avoid returning an invalid modified value (false) for file-backed templates by returning null instead, and aligns the REST schema/tests with that behavior.

Changes:

• Return null for modified when a template has no modification date (e.g., file-backed templates).
• Update the modified schema type to allow null (string|null).
• Add PHPUnit coverage for the modified response value and schema.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/phpunit/tests/rest-api/wpRestTemplatesController.php	Adds tests asserting file-backed templates expose modified as null and that the schema allows null.
src/wp-includes/rest-api/endpoints/class-wp-rest-templates-controller.php	Returns null for empty modified and updates the modified schema type to `string

Comments suppressed due to low confidence (1)

src/wp-includes/rest-api/endpoints/class-wp-rest-templates-controller.php:791

• date is documented in the schema as string|null, but file-backed templates set $template->date to null (see _build_block_template_result_from_file()), which makes mysql_to_rfc3339() return false. That means the REST response can still violate the schema for file-backed templates.

		if ( rest_is_field_included( 'date', $fields ) ) {
			$data['date'] = mysql_to_rfc3339( $template->date );
		}

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[ntsekouras]

Changes will be included here: #12709