GHSA-m6vc-f87m-cc2h

### Impact

The `DynamicClientRegistrationController#register` action hard-codes `confidential: false` when creating applications (dynamic_client_registration_controller.rb:18-25), yet the response includes a client_secret and advertises `token_endpoint_auth_methods_supported: ["client_secret_basic", "client_secret_post"]`.

Because Doorkeeper's `Application.by_uid_and_secret` treats a blank/missing secret as valid for non-confidential (public) clients, an attacker who knows only the client_id (which is public information) can authenticate as the dynamically-registered client at the token endpoint.

**Note** that Dynamic Client Registration is opt-in feature which is disabled by default so only projects that explicitly enabled it are affected.

**Steps to Reproduce**

1. Enable dynamic client registration in the initializer 2. POST /oauth/registration with client_name, redirect_uris, and scope 3. Observe: response returns client_secret, but the created Doorkeeper::Application has confidential: false 4. Call `Doorkeeper::Application.by_uid_and_secret(client_id, nil)` — it returns the application (credentials bypass) 5. POST /oauth/token with grant_type=client_credentials and only client_id (no client_secret) — the token endpoint issues an access token without any secret verification

### Patches Patched in 1.10.0

### Workarounds Upgrade existing applications created with a Dynamic Client registration to have `confidential: true`