Pre-check a custom MCP server's OAuth/DCR support

Body for POST /v1/integrations/connections/custom-mcp/discover.

A read-only pre-check (no DB row) so the connect UI can detect whether a server supports OAuth Dynamic Client Registration before asking for a client_id. transport is accepted for symmetry with the connect body but is advisory — discovery probes well-known HTTP endpoints transport-agnostically.

Returned by POST /v1/integrations/connections/custom-mcp/discover.

supports_dcr is the signal the UI needs: True ⇒ hide the manual client_id fields (the server advertises an RFC 7591 registration endpoint); False ⇒ no discoverable metadata, so the user must supply a client_id.