MOD-14618 Fix race condition when calling indexLabelCount on SVS without locks (#919)

* proposal to fix flaky test

* clarify scope

* fiox preferAdHocSearch and add comment to indexLabelCount()

* revert preferAdHocSearch

---------

Co-authored-by: meiravgri <meirav.grimberg@redis.com>
(cherry picked from commit 42fdc90)

Author: JoanFM

src/VecSim/vec_sim_tiered_index.h

@@ -420,14 +420,22 @@ VecSimDebugInfoIterator *VecSimTieredIndex<DataType, DistType>::debugInfoIterato
                          .fieldType = INFOFIELD_UINT64,
                          .fieldValue = {FieldValue{.uintegerValue = info.tieredInfo.bufferLimit}}});
 
-    infoIterator->addInfoField(VecSim_InfoField{
-        .fieldName = VecSimCommonStrings::FRONTEND_INDEX_STRING,
-        .fieldType = INFOFIELD_ITERATOR,
-        .fieldValue = {FieldValue{.iteratorValue = this->frontendIndex->debugInfoIterator()}}});
+    // Acquire shared locks to safely access each sub-index's debug info during background indexing.
+    // Only hold each lock while accessing its respective index to minimize contention.
+    {
+        std::shared_lock<std::shared_mutex> flat_lock(this->flatIndexGuard);
+        infoIterator->addInfoField(VecSim_InfoField{
+            .fieldName = VecSimCommonStrings::FRONTEND_INDEX_STRING,
+            .fieldType = INFOFIELD_ITERATOR,
+            .fieldValue = {FieldValue{.iteratorValue = this->frontendIndex->debugInfoIterator()}}});
+    }
 
-    infoIterator->addInfoField(VecSim_InfoField{
-        .fieldName = VecSimCommonStrings::BACKEND_INDEX_STRING,
-        .fieldType = INFOFIELD_ITERATOR,
-        .fieldValue = {FieldValue{.iteratorValue = this->backendIndex->debugInfoIterator()}}});
+    {
+        std::shared_lock<std::shared_mutex> main_lock(this->mainIndexGuard);
+        infoIterator->addInfoField(VecSim_InfoField{
+            .fieldName = VecSimCommonStrings::BACKEND_INDEX_STRING,
+            .fieldType = INFOFIELD_ITERATOR,
+            .fieldValue = {FieldValue{.iteratorValue = this->backendIndex->debugInfoIterator()}}});
+    }
     return infoIterator;
 };