Options for the MongoAdapter — the 'mongodb' arm.

A union of two arms, so supplying neither url nor client is a compile error rather than a connect() throw — the guarantee every other built-in arm gives ('prisma' requires prismaClient, 'drizzle' requires both Drizzle options, 'custom' requires adapter). Both members stay readable on either arm, so the adapter reads options.url and options.client without narrowing first.