xds/clusterimpl: fix uint32 overflow computing EDS drop ratio

[nvxbug]

handleClusterConfigLocked turns each EDS drop_overload into a per-million rate with Numerator * million / Denominator in uint32. The numerator comes from the control plane and reaches a million for a 100% drop, so with the million denominator the product overflows a uint32 once the numerator hits 4295 (a 0.43% drop), and the rate then collapses toward zero: a 50% drop computes to 0.18% and a 100% drop to 0.36%. A fraction above 100% instead lands RequestsPerMillion above a million, which underflows million - RequestsPerMillion in newDropper.

Move the computation into dropRequestsPerMillion, which multiplies in uint64 and caps the ratio at a million. Keeping the clamp next to the arithmetic that builds DropConfig leaves the raw numerator and denominator on the parsed resource and applies drops at the configured rate.

RELEASE NOTES:

• xds: fix xDS EDS drop policies being applied at a much lower rate than configured due to an integer overflow

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.10%. Comparing base (75f3c0b) to head (3be127e).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9257      +/-   ##
==========================================
- Coverage   83.23%   83.10%   -0.13%     
==========================================
  Files         421      422       +1     
  Lines       34086    34833     +747     
==========================================
+ Hits        28370    28948     +578     
- Misses       4281     4394     +113     
- Partials     1435     1491      +56     

Files with missing lines	Coverage Δ
internal/xds/balancer/clusterimpl/clusterimpl.go	88.46% <100.00%> (+0.20%)	⬆️

... and 34 files with indirect coverage changes

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[easwars]

Moving to @mbissa for second set of eyes