management_api: fail closed on ambiguous connection options oneOf

CreateConnectionReqOptions, ReplaceConnectionReqOptions and
UpdateConnectionReqOptions silently defaulted to variant 0 when no
discriminator key matched (!found), instead of erroring. With variants
now sharing keys like is_trusted/is_use_custom_domain, an ambiguous or
malformed payload could get silently decoded as the wrong connection
type instead of failing. Restored the fail-fast error.

Author: dtoxvanilla1991