fix: Use template title and category slugs in pattern headers

[ishitaj34]

What?

Closes #871

Fixes two issues in the pattern file header generation:

1. pattern_from_template was using the template slug as the title instead of the human-readable $template->title label shown in the Site Editor.
2. pattern_from_wp_block was writing pattern category names into the Categories header instead of taxonomy slugs.

Why?

For example, when a user exported a theme, the generated pattern .php files had:

• Title: archive-product instead of Title: Product Catalog, making patterns hard to identify in the inserter.
• Categories: Banners instead of Categories: banner, breaking category filtering in the block inserter since Core matches by slug.

How?

In includes/create-theme/theme-patterns.php:

• pattern_from_template: reads $template->title when present, falls back to $template->slug if absent or empty. Applies the existing */ → */ escaping to the title as well.
• pattern_from_wp_block: changes wp_list_pluck( $pattern_category_list, 'name' ) to wp_list_pluck( $pattern_category_list, 'slug' ).

In tests/test-theme-patterns.php:

• test_pattern_from_template_uses_title_not_slug: Checks that the pattern title uses the human-readable title of the template over its slug.
• test_pattern_from_template_falls_back_to_slug_when_title_empty: Checks that an empty or missing title falls back to the slug.
• test_pattern_from_template_escapes_comment_closer_in_title: Checks that */ in the title is safely escaped to */, preventing the docblock of the pattern file from closing prematurely and causing PHP syntax errors or code injection.
• test_pattern_from_wp_block_uses_category_slugs_not_names: Checks that categories writes the category slug instead of its display name.
• test_pattern_from_wp_block_uses_multiple_category_slugs: Checks that multiple categories are exported as comma-separated slugs.

Testing Instructions

Unit tests:

1. Run npm run test:unit:php:setup (starts wp-env).
2. Run npm run test:unit:php:base -- --filter Test_Create_Block_Theme_Patterns.
3. All 27 tests should pass with 0 failures.
4. To specifically run the 5 new tests added above:

npm run test:unit:php:base -- --filter 'test_pattern_from_template_uses_title_not_slug|test_pattern_from_template_falls_back_to_slug_when_title_empty|test_pattern_from_template_escapes_comment_closer_in_title|test_pattern_from_wp_block_uses_category_slugs_not_names|test_pattern_from_wp_block_uses_multiple_category_slugs'

Manual test:

1. Activate a block theme (e.g. Twenty Twenty-Five).
2. For template title: Appearance -> Editor -> Templates. Open any template, and verify it has a human-readable title.
3. For category slugs: Appearance → Editor → Patterns. Create a new pattern assigned to a category.
4. Go to Appearance -> Create Block Theme and export the theme as a zip.
5. Unzip and open the corresponding patterns/*.php. The Title: header should match the Site Editor label, not the file slug, and Categories: should match the file slug.

Use of AI Tools

• AI assistance: Used for drafting tests along with edge cases, and PR description.
• Tool(s) used: Claude Sonnet 4.6