Hide title generation until content exists

[dhrupo]

What?

Closes #391

Hides the Title Generation control until the post has body content, and adds an e2e regression test for that empty-state behavior.

Why?

The current UI shows the title generation toolbar even when there is no content to generate from. That creates a confusing empty-state flow and matches the behavior reported in #391.

How?

The title toolbar now reads live editor state with useSelect and returns null when the edited post content is empty. The existing Generate/Regenerate labeling stays the same once content exists. An e2e test was added to verify the toolbar stays hidden for a blank post and appears after content is added.

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5
Used for: Drafting the implementation and regression test; final changes were reviewed and edited before submission.

Testing Instructions

1. Enable the Title Generation experiment.
2. Create a new post with no title and no body content.
3. Focus the title field and confirm the Title Generation control is not shown.
4. Add body content to the post.
5. Focus the title field again and confirm the control appears with the Generate label.
6. For a post that already has a title and body content, confirm the control still appears with the Regenerate label.

Ran locally:

• npm run typecheck
• npm run lint:js -- src/experiments/title-generation/components/TitleToolbar.tsx tests/e2e/specs/experiments/title-generation.spec.js

Changelog Entry

Fixed - Hide the Title Generation control until a post has content to generate from.

[github-actions]

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Co-authored-by: dhrupo <dhrupo@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: hemant7405 <hdkothari81@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.06%. Comparing base (a04709f) to head (ab80b8f).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #479   +/-   ##
==========================================
  Coverage      69.06%   69.06%           
  Complexity       957      957           
==========================================
  Files             60       60           
  Lines           4512     4512           
==========================================
  Hits            3116     3116           
  Misses          1396     1396           

Flag	Coverage Δ
unit	69.06% <ø> (ø)

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