Inserter: Fix error being thrown for spoken message when inserting default/direct block

[talldan]

What?

Currently the inserter can thrown an error on the navigation block.

Why?

#77877 seems to have unearthed a bug that was present long before that.

Before #77877, allowedBlockType could unusually be set to false, attempting to access allowedBlockType.title would be ok, but the title would be undefined, so users of assistive technologies would hear something like "undefined block added".

After #77877, allowedBlockType can be set to null, attempting to access allowedBlockType.title throws an error. I think the upstream code is better in using more expected types, attempting to access title throws.

How?

The new fix uses getBlockLabel to determine the label of the inserted block, which should be better. To take variations into account, the code also has to replicate what useBlockDisplayTitle does, using getActiveBlockVariation to fall back to the correct variation name.

Testing Instructions

The block I reproduced this on is navigation (inserting a navigation link when clicking the appender). You can't really hear the spoken message using voiceover because there are lots of other announcements at the same time. But I used a breakpoint in the browser dev tools to determine that the message is now "Page link block added".

1. Add a navigation block
2. Stick a breakpoint on the speak( message ) line

In this PR: message is "Page link block added".
In trunk: the code throws an error.

Use of AI Tools

OpenCode / Codex

[github-actions]

Size Change: +129 B (0%)

Total Size: 8.44 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	380 kB	+129 B (+0.03%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 333610b.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/27120283056
📝 Reported issues:

• [Flaky Test] Should save the changes #78689 in /test/e2e/specs/editor/plugins/wp-editor-meta-box.spec.js

[ntsekouras]

Should we move all this logic inside insertOnlyAllowedBlock (with the registry) and calculate on demand?

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: talldan <talldanwp@git.wordpress.org>
Co-authored-by: ntsekouras <ntsekouras@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.