Fixes intermittent error in cypress "Index Text Block" test

[wesleybl]

Waits for the slate editor to be visible before attempting to insert text.

This fixes the error:

  1) Block Indexing Tests
       Index Text Block:
     AssertionError: Timed out retrying after 4000ms: Expected to find content: 'Noam Avram Chomsky' within the element: <div> but never did.
      at Context.eval (webpack://@plone/volto/./cypress/tests/core/blocks/blocks-indexing.js:19:51)

See: https://github.com/plone/volto/actions/runs/13332572803/job/37240061119?pr=6754#step:4:445

Caution

The Volto Team has suspended its review of new pull requests from first-time contributors until the release of Plone 7, which is preliminarily scheduled for the second quarter of 2026.
Read details.

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• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I succesfully ran code linting checks on my changes locally.
• I succesfully ran unit tests on my changes locally.
• I succesfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

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