Zendesk OAuth2 credential cannot connect to OAuth clients marked as Public (PKCE required)

Bug Description The Zendesk OAuth2 API credential locks the grant type to a hidden authorizationCode value. Zendesk enforces PKCE for OAuth clients marked as Public, so connecting fails for these clients. (Marking a client as Public is how Zendesk admins disable the client credentials flow on a shared client.) The generic OAuth2 API credential already exposes the grant type (Authorization Code, Client Credentials, PKCE), and n8n's OAuth core supports all three flows. The Zendesk credential just doesn't expose the choice. To Reproduce 1. In Zendesk Admin Center, go to Apps and integrations > APIs > Zendesk API > OAuth Clients and create an OAuth client marked as Public. 2. In n8n, create a Zendesk OAuth2 API credential with the subdomain, client ID, and client secret, and click Connect my account. 3. The authorization fails: Zendesk requires a PKCE code challenge for public clients, and there's no way to select PKCE in the credential. Expected behavior The…