Replace EverblockPage selector with select choices; add cover image & excerpt override

[CySSoO]

Motivation

• The PrettyBlocks selector referencing the EverblockPage object model is not compatible with the selector system and must be replaced with a static select choices list.
• Provide editors the ability to override the guide cover image and the excerpt/CTA text per state.
• Prefer using a custom cover image set on the block state before falling back to the Everblock page cover.
• Improve labels to make intent clearer by renaming Summary to Excerpt in the Guide repeater configuration.

Description

• Added getEverblockPageChoices(Context $context, Module $module) which builds a choices array from \EverblockPage::getPages() and returns a default empty option.
• Replaced the selector field for guide with a select using choices => $everblockPageChoices in src/Service/EverblockPrettyBlocks.php.
• Added a cover_image fileupload field and changed the summary field label to Excerpt in the Guide repeater configuration.
• Updated views/templates/hook/prettyblocks/prettyblock_guides_selection.tpl to prefer $state.cover_image (state override) and only fall back to the page's getCoverImageData() when no state cover exists, plus use the state summary with fallback to page short description.

Testing

• No automated tests were executed for this change.
• The change compiles (PHP syntax checks were not explicitly run as part of this rollout).
• Manual inspection of the updated template and service code was performed during the change.
• Further QA or automated tests are recommended to validate runtime behavior in the CMS and block editor.

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Codex Task