[t-hamano]

Backport of the PHP part of WordPress/gutenberg#78367. Supersedes #13953.

When the filesystem method requires credentials that are not stored (for example, FS_METHOD is ftpext), installing a plugin from Settings > Connectors fails because the REST plugins endpoint rejects the request with fs_unavailable. With the Gutenberg PR, the Connectors JS falls back to wp.updates.installPlugin() in that case, which shows the "Connection Information" modal. This PR loads what that fallback needs on the Connectors screen: the updates script, the credentials modal, and the admin notice templates, the same way Plugins > Add Plugin does.

This has no effect until the Connectors JS from the Gutenberg PR is synced to core.

Testing Instructions

Requires the Connectors JS from the Gutenberg PR.

1. Add define( 'FS_METHOD', 'ftpext' ); to wp-config.php without storing FTP credentials.
2. Go to Settings > Connectors and click Install for a connector whose plugin is not installed.
3. The "Connection Information" modal opens, and valid credentials install and activate the plugin.

Trac ticket: https://core.trac.wordpress.org/ticket/65223

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Porting the PHP change from the Gutenberg PR and drafting this PR description; reviewed by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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