Fix comments UI nullable reply HTML handling

[PedroMarianoAlmeida]

Why I am making it

This started as a comments-ui typecheck cleanup. Comment.html is typed as string | null, because deleted comment replies can be returned with html: null when they need to stay visible as tombstones for visible descendant replies.

Some reply paths still treated comment.html as always being a string when rendering or building the “reply to” snippet. This makes those paths narrow nullable HTML explicitly and adds regression coverage for the deleted reply tombstone case.

What it does

• Narrows nullable comment HTML before rendering comment content.
• Avoids generating a reply snippet from a reply when its HTML is null.
• Only renders ReplyFormBox when an openForm exists.
• Adds a comments-ui E2E regression test for a deleted reply with html: null and a visible child reply.
• Verifies the deleted reply renders as a tombstone without comment content, reply action, or like action.

Type errors fixed

This removes the apps/comments-ui/src/components/content/comment.tsx errors caused by nullable comment HTML and optional reply form state:

• Argument of type 'Comment' is not assignable to parameter of type '{ html?: string | undefined; }'
• Type 'string | null' is not assignable to type 'string | undefined'
• Type 'OpenCommentForm | undefined' is not assignable to type 'OpenCommentForm'
• Type 'string | null' is not assignable to type 'string'

Why developers need it

This cleans up the comments-ui typecheck path and protects the threaded comments UI from crashing or exposing actions for deleted reply tombstones when API data contains html: null.

Testing

• rtk pnpm --filter @tryghost/comments-ui test:acceptance test/e2e/content.test.ts
• rtk pnpm --dir apps/comments-ui exec eslint --cache -- test/e2e/content.test.ts
  • Exits 0 with the existing warning that the test file is ignored because comments-ui lint config only targets src.

Checklist

• I've read and followed the Contributor Guide
• I've explained my change
• I've written an automated test to prove my change works

[coderabbitai]

Walkthrough

Comment rendering now treats missing HTML as absent content: opening the inline reply form is blocked when a published comment has no HTML, published bodies render only when HTML exists, and both published and unpublished layouts require an open form before showing the reply form. The e2e suite adds coverage for a deleted reply with null HTML rendering as a tombstone without action controls while a nested visible child reply remains visible.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: handling nullable reply HTML in the comments UI.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description matches the code changes and added regression test for nullable comment HTML and deleted reply tombstones.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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