Always guess position with a chance.

This match the function name and the behavior of other engines.

PiperOrigin-RevId: 936051384

fuzztest/internal/domains/container_mutation_helpers.h

diff --git a/fuzztest/internal/domains/container_mutation_helpers.h b/fuzztest/internal/domains/container_mutation_helpers.h
index f17d52f..530b523 100644
--- a/fuzztest/internal/domains/container_mutation_helpers.h
+++ b/fuzztest/internal/domains/container_mutation_helpers.h
@@ -101,7 +101,8 @@
 
 inline size_t GetOrGuessPositionHint(std::optional<size_t> position_hint,
                                      size_t max, absl::BitGenRef prng) {
-  if (position_hint.has_value()) {
+  if (position_hint.has_value() && *position_hint <= max &&
+      absl::Bernoulli(prng, 0.5)) {
     return *position_hint;
   } else {
     return ChooseOffset(max + 1, prng);