[ZEPPELIN-6544] Harden keyboard-shortcut e2e helpers against silent passes

[voidmatcha]

What is this PR for?

Follow-up to ZEPPELIN-6536 (#5304). A cross-review of the merged keyboard-shortcut suite found four silent-pass risks in the helpers; this PR closes them:

• pressShortcutFromHostUntil now polls for the press's effect before retrying, so a slow toggle is not double-pressed and the loop cannot return on the transient state between two in-flight effects.
• The clear-output test asserts the result is visible before pressing; previously a no-output run let the helper skip the press entirely.
• InsertAbove/InsertBelow emptiness assertions now gate on the editor being rendered (waitForEditorRendered); an unrendered editor reads as '' and matched vacuously.
• setCodeEditorContent compares normalized equality instead of containment, so stale fixture text fails at seeding time.

Test-only change; no product code touched.

What type of PR is it?

Improvement

What is the Jira issue?

ZEPPELIN-6544

How should this be tested?

Questions:

• Does the license files need to update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No