Fix peak RSS calculation on macOS in FuzzTest watchdog.

On macOS (`__APPLE__`), `getrusage(RUSAGE_SELF, &usage)` reports `ru_maxrss` in bytes rather than kilobytes (KiB) as on Linux. Previously, `GetPeakRSSBytes()` unconditionally multiplied `usage.ru_maxrss` by 1024, overreporting peak RSS usage by 1024x on macOS and causing FuzzTest targets to abort immediately when a non-zero RSS limit was configured.

Return `usage.ru_maxrss` directly on macOS in `GetPeakRSSBytes()` so watchdog RSS limit checks compare accurate byte counts across platforms.

PiperOrigin-RevId: 998805274

fuzztest/internal/runtime.cc

diff --git a/fuzztest/internal/runtime.cc b/fuzztest/internal/runtime.cc
index 2eb4f28..5df7ea7 100644
--- a/fuzztest/internal/runtime.cc
+++ b/fuzztest/internal/runtime.cc
@@ -436,9 +436,14 @@
 #else
   struct rusage usage = {};
   if (getrusage(RUSAGE_SELF, &usage) != 0) return 0;
-  // On Linux, ru_maxrss is in KiB
+#ifdef __APPLE__
+  // On MacOS, ru_maxrss is in bytes.
+  return usage.ru_maxrss;
+#else
+  // On Linux, ru_maxrss is in KiB.
   return usage.ru_maxrss * 1024;
 #endif
+#endif
 }
 
 void Runtime::CheckWatchdogLimits() {