Connectors: Add connector registry for extensibility

Introduces `WP_Connector_Registry` class and a `wp_connectors_init` action hook so plugins can register their own connectors alongside the built-in defaults (Anthropic, Google, OpenAI).

Key changes:
* `WP_Connector_Registry` — A `final` singleton class managing connector registration and lookup, with validation for IDs, required fields, and authentication methods.
* `wp_connectors_init` action — Fired during `init` after built-in connectors are registered. Passes the registry instance so plugins call `$registry->register()` directly.
* `_wp_connectors_init()` — Private function that creates the registry, merges hardcoded defaults with AI Client registry data, registers them, then fires the action.
* Public read-only functions — `wp_is_connector_registered()`, `wp_get_connector()`, `wp_get_connectors()` for querying the registry after initialization.
* Logo URL support — Connectors can include an optional `logo_url` field resolved from plugin directories via `_wp_connectors_resolve_ai_provider_logo_url()`.
* Timing guards — `set_instance()` rejects calls after `init` completes. Registration is only possible during `wp_connectors_init`.
* Connector API key settings are now only registered when the provider exists in the AI Client registry.
* Refactors `_wp_connectors_get_connector_settings()` to read from the registry via `wp_get_connectors()`.

Developed in https://github.com/WordPress/wordpress-develop/pull/11175

Props gziolo, flixos90, mukesh27, westonruter.
Fixes #64791.




git-svn-id: https://develop.svn.wordpress.org/trunk@61943 602fd350-edb4-49c9-b593-d223f7449a82