feat(blocks): add accordion block for collapsible nested content

[all3f0r1]

What does this PR do?

Adds a new accordion Block Kit block that wraps nested blocks under a labeled, expandable trigger. Maps directly to Kumo's Collapsible component. Plugin admin pages can now hide secondary content (advanced settings, FAQs, auxiliary panels) without paginating or round-tripping through block_action.

Open/closed state is local to the rendered React component (with optional default_open) — no new interaction type needed.

Discussion: #789

Type of change

• Bug fix
• Feature (requires maintainer-approved Discussion)
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes (for @emdash-cms/blocks — the only package touched. A pre-existing typecheck failure in @emdash-cms/admin/src/components/PortableTextEditor.tsx exists on main and is unrelated to this PR.)
• pnpm lint passes (lint:json diagnostic count unchanged from baseline)
• pnpm test passes (@emdash-cms/blocks: 75 tests, all green)
• pnpm format has been run
• I have added/updated tests for my changes (validation: 1 valid + 3 invalid cases; renderer: 2 cases covering toggle behavior and nested onAction propagation)
• User-visible strings in the admin UI are wrapped for translation — N/A, no admin UI strings introduced
• I have added a changeset (@emdash-cms/blocks minor — additive)
• New features link to an approved Discussion: Add an `accordion` Block Kit block (collapsible nested content) #789

AI-generated code disclosure

• This PR includes AI-generated code

Screenshots / test output

✓ tests/validation.test.ts (48 tests) — includes new accordion valid + 3 invalid cases
✓ tests/renderer.test.tsx (21 tests) — includes new accordion default-closed + default_open + onAction propagation cases
✓ tests/form-conditions.test.tsx (6 tests)

Test Files  3 passed (3)
     Tests  75 passed (75)

Design notes

• Naming: chose accordion over collapsible (the Kumo component name) because plugins typically stack several of them — "accordion" is the more familiar UX term. Easy to flip if maintainers prefer the Kumo name.
• No state round-trip: open/closed is local React state. default_open covers the "should this start expanded" case; rejected a block_action-based reactive variant as adding round-trips per toggle for negligible benefit.
• Empty blocks allowed: mirrors how an empty section is allowed; no minimum length enforced.
• Recursive validation: walks nested blocks so an invalid child reports the correct path (e.g. blocks[0].blocks[2].text).

[changeset-bot]

🦋 Changeset detected

Latest commit: 7439fe6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@emdash-cms/blocks	Minor
@emdash-cms/admin	Minor
emdash	Minor
@emdash-cms/plugin-embeds	Patch
@emdash-cms/cloudflare	Minor
@emdash-cms/fixture-perf-site	Patch
@emdash-cms/perf-demo-site	Patch
@emdash-cms/cache-demo-site	Patch
@emdash-cms/auth	Minor
@emdash-cms/gutenberg-to-portable-text	Minor
@emdash-cms/x402	Minor
create-emdash	Minor
@emdash-cms/auth-atproto	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

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commit: 7439fe6

[ascorbic]

/review

[ask-bonk]

LGTM!

github run

[github-actions]

Overlapping PRs

This PR modifies files that are also changed by other open PRs:

• feat(admin): add media_picker BlockKit element #731 (4 shared files)
• feat(blocks): add Tab block to Block Kit #717 (5 shared files)

This may cause merge conflicts or duplicated work. A maintainer will coordinate.

[ascorbic]

@all3f0r1 the conflicts are all just with your other PR I think, and should be quite easy to resolve

[all3f0r1]

Mergeable now.

[ascorbic]

Thanks!