Tiptap RTE: Adds support for Reusable Content of Blocks - #23195

… entries

The umb-entity-frame label tab clips when a block is at the top of the
RTE without this spacing.

Reduce cognitive complexity of #restoreUnusedBlocks by extracting
content/settings restoration into helper methods, use .includes()/Set.has()
over indexOf()/Array.includes(), and use RegExp.exec() over String.match().

Adds a sibling test to the incoming v19/dev doc-walk test, asserting
the walk also finds layout keys (including the legacy data-key-missing
fallback), matching how #updateBlocks now discovers blocks post-merge.

Block RTE, Grid, List and Single entry elements each duplicated an
identical #updateExposedState() method and its three backing
observers. Move the derivation onto UmbBlockEntryContext as a single
mergeObservables-based isExposed observable, consumed the same way by
all four elements. Fixes a SonarQube new-code duplication gate failure
on block-rte-entry.element.ts.

These two cases were merged up from v17/dev before RTE blocks gained a
data-key layout attribute alongside data-content-key, so their
expected markup no longer matches the block node's actual (correct)
output. Use distinct data-key/data-content-key values to assert the
div-nesting behaviour without relying on the legacy same-value
back-fill, which is already covered by block.tiptap-extension.test.ts.

BlockRegex required data-content-key to appear immediately after the
tag name (or an optional class attribute), so it never matched once
data-key started being emitted before data-content-key. That left
RTE blocks rendering as unreplaced placeholders on the front end
after being re-saved through the editor. Widen the pattern to accept
attributes on either side of data-content-key.

…te-blocks

…l blocks

…UnusedBlocks

to update when UFM label has resolved.

…work to their own PRs

Server-side data-key parsing/stripping, the shared isExposed
derivation on UmbBlockEntryContext, and the RTE entry umb-entity-frame
UI are independent of the layout-key rework and are easier reviewed
separately. Moved to:

- v19/bugfix/rte-block-data-key-parsing
- v19/improvement/block-entry-is-exposed
- v19/feature/rcob-rte-block-entry-ui

This branch now covers only identifying RTE blocks by layout key.

…dering test

The three existing assertions read element.value, which only re-syncs
from manager state via a property-context-dependent observer that this
fixture (no real UMB_PROPERTY_CONTEXT provider) never wires up, so
they silently asserted on stale data regardless of the code under
test. Assert against the manager context directly instead — that's
the actual contract _filterUnusedBlocksByLayoutKeys owns.

Also adds a test pinning the layout-before-content/settings removal
order, which nothing previously guarded: reverting the reorder in
rte-base.element.ts turned none of the existing tests red.