[pfefferle]

Proposed changes:

Adds a minimal version of FEP-65ca: Common Client Configuration, so C2S clients can look up limits and account pages before they post. The FEP is still a draft, so I only added values that are true for every WordPress site today.

• /.well-known/activitypub-client-config returns the discovery document, with a link to /activitypub/1.0/client-config/1.0.
• The configuration document has composer.max_length: null, accounts.username.changeable: false and the account pages: login, password_reset and account_settings, plus registration and privacy if the site has them.
• Both routes only exist when the ActivityPub API is enabled.
• No media section yet. It can be added once uploads land (Add C2S media upload (uploadMedia endpoint) #3287).

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Enable the ActivityPub API in Settings → ActivityPub → Advanced.
• Open /.well-known/activitypub-client-config and check that the link points to /wp-json/activitypub/1.0/client-config/1.0.
• Open that link and check the links object.
• Enable "Anyone can register" in Settings → General, and check that registration shows up.
• Disable the ActivityPub API, and check that both URLs return a 404.

Changelog entry

Entry lives in .github/changelog/add-fep-65ca-client-config.