get_block_templates(): Fix non-sequential array keys for registry-only template matches

[im3dabasia]

Trac ticket:

What Why How

get_block_templates() returns an array whose keys are not sequential when the only matching template comes from the plugin template registry (WP_Block_Templates_Registry), which stores templates keyed by name. When no wp_template post or theme file also matches, array_merge() carries those string keys straight into the result with no numeric 0 index.

This breaks callers that assume a plain list, e.g. wp_get_post_content_block_attributes() in block-editor.php, which accesses $templates[0]. Reproduced in WordPress/gutenberg#80291 : assigning a plugin-registered template to a post and opening it in the editor triggers PHP warnings and deprecation notices.

Two tests added:

• test_get_block_templates_returns_sequentially_indexed_array_for_registry_only_match - asserts get_block_templates() returns a sequentially indexed array when the registry is the sole match.
• test_wp_get_post_content_block_attributes_with_registry_only_assigned_template - reproduces the original issue end-to-end.

Before

both the tests I added were failing locally.

After

Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Model(s): Claude Sonnet 4.5

[github-actions]

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