Add support for Template Activation

We should add support for Template Activation, which is currently under an experiment in Gutenberg: https://github.com/WordPress/gutenberg/pull/73252

When this is stabilised, it will likely land in WordPress 7.0, so we should make sure the plugin works with this new template functionality before then.

Tasks

• Update get_block_templates() calls to explicitly request inactive templates when needed (e.g., when clearing customisations or exporting all templates): https://github.com/WordPress/create-block-theme/pull/798
• Modify clear_user_templates_customizations() and clear_user_template_parts_customizations() to include inactive templates: https://github.com/WordPress/create-block-theme/pull/798
• Add activation state checking to template filtering logic by modifying should_include_template()
• Add options to control whether inactive templates are included in exports
• Consider how to handle duplicate templates (same slug, different IDs) when saving to theme files

Start by reviewing the Gutenberg Template Activation experiment and the referenced create-block-theme PR 798. Trace get_block_templates(), clear_user_templates_customizations(), clear_user_template_parts_customizations(), and should_include_template() to understand inactive-template handling. Done means exports, filtering, clearing customizations, and duplicate-template behavior follow the agreed activation rules.