xds: reject EDS drop policy with unsupported denominator

[nvxbug]

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.30%. Comparing base (26ffdb3) to head (4eabde4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9218      +/-   ##
==========================================
+ Coverage   83.22%   83.30%   +0.08%     
==========================================
  Files         420      420              
  Lines       34024    34030       +6     
==========================================
+ Hits        28317    28350      +33     
+ Misses       4275     4256      -19     
+ Partials     1432     1424       -8     

Files with missing lines	Coverage Δ
...nternal/xds/xdsclient/xdsresource/unmarshal_eds.go	96.22% <100.00%> (+0.14%)	⬆️

... and 29 files with indirect coverage changes

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