Separator Block: Apply default block variation when inserting via --- shortcut

[BugReportOnWeb]

What?

Closes #75259

Apply default block variation attributes when creating a Separator block via the --- markdown shortcut.

Why?

When a default block variation is registered for the Separator block, it is correctly applied when inserting via the block inserter UI. But when inserting via the --- shortcut, it creates the Separator block with no attributes, so the default variation is never used and the block falls back to its hardcoded defaults.

How?

Updated the input transform to check the registered default variation for the Separator block before calling createBlock, and merge its attributes in.

If no default variation is registered, attributes falls back to an empty object and createBlock behaves exactly as before with no registered default variation.

Testing Instructions

1. Register a default block variation for the Separator with tagName: 'div' (could use the snippet from the issue)
2. Insert a Separator via the block inserter and observe it uses div
3. Type --- to insert a Separator. Before the fix it uses hr, after the fix it correctly uses div.
4. Remove the default variation registration and repeat step 3. Separator should still insert correctly with default hr behaviour.

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: BugReportOnWeb <devasheeshkaul@git.wordpress.org>
Co-authored-by: Mustafabharmal <mustafabharmal@git.wordpress.org>
Co-authored-by: mrwweb <mrwweb@git.wordpress.org>

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