xds: reject EDS drop policy with unsupported denominator (#9218)

`parseDropPolicy` maps a `FractionalPercent` denominator to a divisor
with a switch that has no default, so an EDS `drop_overload` carrying an
unknown denominator enum value leaves the divisor at 0. That value
reaches `clusterimpl`, which evaluates `d.Numerator * million /
d.Denominator`, and a management server sending such a resource crashes
the client with an integer divide-by-zero.

Reject the unsupported denominator while parsing the resource so it is
NACKed, matching how `parseEDSRespProto` already errors on other
malformed fields. Validating in the parser keeps the balancer free of
divisor checks and stops one bad resource from taking down the client.

RELEASE NOTES:
- xds: Prevent divide-by-zero client crashes on unsupported EDS drop
policy denominators