Tiptap RTE: Add delete action support for RTE blocks (closes #21345)

[leekelleher]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Adds a delete button to RTE block entries that removes blocks from both the editor HTML and the block manager data. Implements an HTML-first deletion approach that enables Ctrl+Z undo support by leveraging the existing _filterUnusedBlocks mechanism.

This is a partial fix towards #21345.

Changes

• block-rte-entry.element.ts: Add delete button to action bar
• block-rte-manager.context.ts: Add pendingDeletions state and methods for HTML-first deletion flow
• block-rte-entries.context.ts: Override delete() to use pending deletion mechanism
• block.tiptap-api.ts: Add observer to process pending deletions and remove blocks from editor via ProseMirror transactions

How it works

1. User clicks delete button → confirmation modal appears
2. After confirmation, contentKey is added to pendingDeletions on the manager
3. Tiptap API observes pendingDeletions → removes HTML from editor (creating undo history)
4. Tiptap's onUpdate fires → _filterUnusedBlocks runs
5. _filterUnusedBlocks stores block data in lookup maps (enabling undo) and removes from manager
6. User can press Ctrl+Z → Tiptap restores HTML → _restoreUnusedBlocks restores data

How to test

1. Create a document type with an RTE property that has blocks enabled
2. Add a block to the RTE
3. Hover over the block to see the action bar
4. Click the delete button (trash icon)
5. Confirm the deletion in the modal
6. Verify: Block should disappear from the editor
7. Press Ctrl+Z (undo)
8. Verify: Block should reappear with its data restored
9. Save and reload to verify persistence

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