cms-admin: Add isTextBlockType option to createTipTapRichTextBlock

[VPS-Andreas]

Summary

The text block style dropdown was the only way to apply a style like a large "Display" heading variant, even for blocks where that style is really a distinct, editor-facing block type in its own right — some Draft.js RTEs had exactly this: a display block type sitting above header-one, both rendering as <h1>, display a large hero-style variant. That produced two dropdowns where the legacy block had one, and left the style dropdown showing only "Default"/"Heading N" for every other heading level.

A textBlockStyles entry with isTextBlockType: true now shows up as its own entry in the text block type dropdown instead, right above the plain heading level entry it shares a tag with. Picking that plain entry, or any other tag, always clears the style, so it can't linger as a stale value. Requires appliesTo to name exactly one tag, and that tag can't also carry a defaultTextBlockStyles entry, since the two would fight over what "no explicit style" means for it.

Example

createTipTapRichTextBlock({
    paragraph: false,
    heading: { levels: [1, 2, 3, 4, 5] },
    textBlockStyles: [
        {
            name: "display",
            label: "Display",
            appliesTo: ["heading-1"],
            isTextBlockType: true,
            element: (props) => <h1 style={{ fontSize: 56 }} {...props} />,
        },
    ],
});

The type dropdown becomes Display, Heading 1, Heading 2, Heading 3, Heading 4, Heading 5 — matching the legacy single-dropdown block type select.

Stacking note

⚠️ Based on #6366, not main — that PR needs to merge first. Once it does:

1. git rebase --onto <new main tip> <old tiptap-default-text-block-style tip> tiptap-text-block-type-select-style (same dance already done twice this session for Support heading-only TipTap rich text blocks #6240→Add defaultTextBlockStyles option to createTipTapRichTextBlock #6366 and main→cms-admin: Fix invalid DOM nesting in textBlockStyles node views #6363/cms-admin: Don't insert a trailing paragraph after a heading #6364)
2. Switch this PR's base to main in GitHub (or verify GitHub already auto-retargeted it after the base branch was deleted — still re-check the diff either way, since a squash-merge rewrites commit SHAs)
3. Re-verify tsc/eslint/the Storybook story after the rebase

Test plan

• tsc and eslint pass for cms-admin
• New Storybook story (blocks/TipTapRichTextBlock → "Is Text Block Type") verified interactively via Playwright — dropdown order (Display above Heading 1), no separate style dropdown, clearing on switch to the plain entry and to another tag: https://69df3371c46abe69b5199825-bqiylrjspn.chromatic.com/?path=/story/blocks-tiptaprichtextblock--is-text-block-type
• Still marked draft: blocked on Add defaultTextBlockStyles option to createTipTapRichTextBlock #6366 merging first (see Stacking note)

🤖 Generated with Claude Code