Fix MutationObserver crash when editor iframe body isn't ready

[vedant-ere]

Summary

The title toolbar's MutationObserver setup waited a fixed 500ms before assuming the editor canvas iframe was ready, then called observer.observe(editorDoc.body, ...). The iframe element (and its contentDocument) can exist before the document's body does, so on slower editor boots this threw an uncaught TypeError. Because observer was assigned before the throwing call, the setup never ran again, which also silently broke the toolbar re-attach behavior from #694 for that session.

Fix

• Check that editorDoc.body actually exists before calling observe().
• If it's not ready yet, retry every 200ms (capped at 20 tries) instead of relying on one fixed delay.
• Only assign the observer once observe() has succeeded, so retries stay possible.
• Clean up the retry timer on unmount.

Fixes #833

Test plan

• Enable Title Generation experiment, open the block editor, confirm no console errors
• Verify toolbar still appears/hides correctly on title focus/blur
• Toggle "Show template" off and back on, confirm toolbar re-attaches (per Fix: Title Generation: toolbar button disappears after toggling "Show template" off (normal editing mode), until reload #694)

[github-actions]

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Co-authored-by: vedant-ere <vedantere@git.wordpress.org>
Co-authored-by: soydiloreto <pablodiloreto@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.74%. Comparing base (cf0cb09) to head (f35684f).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #849   +/-   ##
==========================================
  Coverage      75.74%   75.74%           
  Complexity      2088     2088           
==========================================
  Files             99       99           
  Lines           8630     8630           
==========================================
  Hits            6537     6537           
  Misses          2093     2093           

Flag	Coverage Δ
unit	75.74% <ø> (ø)

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