Fix OOB read in FailoverErrorsSettingsBase::shouldFailover (MACA T271785672)

Summary:
`memcache/mcrouter-fbpkg` R5519-R5521 fail at the Contbuild Tracking Node: lionhead harness `CarbonResultReproFuzzerd4` reproduces a heap out-of-bounds read in `FailoverErrorsSettingsBase::List::shouldFailover`. A malicious memcache backend sends a Caret reply whose `result` field is an int16 outside `[0, NUM_RESULTS)` (`CarbonProtocolReader` casts the wire value to `carbon::Result` unchecked); `shouldFailover` then indexes `failover_[result]` — a `std::array<bool, 36>` — out of bounds. Fix: bounds-check the index before the array read, falling back to `isFailoverErrorResult` (safely returns false for unknown results). Valid results are unchanged.

Sentinel-Harness: claude

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Model used: Claude Opus 4.8

Differential Revision: D112691076

fbshipit-source-id: 35ab5dbeae78da9a56b8d62649af6e5747483dde

Author: generatedunixname26860585020226206

mcrouter/lib/FailoverErrorsSettingsBase.cpp

@@ -47,8 +47,10 @@ FailoverErrorsSettingsBase::List::List(const folly::dynamic& json) {
 
 bool FailoverErrorsSettingsBase::List::shouldFailover(
     const carbon::Result result) const {
-  if (failover_ != nullptr) {
-    return (*failover_)[static_cast<size_t>(result)];
+  const auto index = static_cast<size_t>(result);
+  if (failover_ != nullptr &&
+      index < static_cast<size_t>(carbon::Result::NUM_RESULTS)) {
+    return (*failover_)[index];
   }
   return isFailoverErrorResult(result);
 }