Fix schema version mismatch for index restore with AtTable operation

Problem:
- After restoring index impl tables, schema version mismatch occurred
- Index metadata expected version X but impl table had version X+1
- Tests failed with: "schema version mismatch during metadata loading"

Root Cause:
- Index impl table restore incremented the impl table's schema version
- Parent index metadata schema version was not updated
- This caused a mismatch when querying the restored table

Solution:
- Added AtTable operation at the index path after impl table restore
- Mirrors CDC stream pattern (schemeshard__operation_backup_backup_collection.cpp:218)
- AtTable operation syncs index metadata schema version with impl table
- Ensures index object and impl table stay in sync

The fix creates two operations per index restore:
1. Restore impl table (increments impl table schema version)
2. AtTable at index path (syncs index metadata schema version)

This matches how CDC streams handle the same issue when creating
streams on index impl tables during backup operations.

Author: claude

ydb/core/tx/schemeshard/schemeshard_incremental_restore_scan.cpp

@@ -857,6 +857,41 @@ void TSchemeShard::CreateSingleIndexRestoreOperation(
     // Send the request (parallel with table operations)
     LOG_I("Sending index restore operation for: " << dstIndexImplPath);
     Send(SelfId(), indexRequest.Release());
+
+    // Create AtTable operation to sync index schema version with impl table
+    // (similar to CDC stream pattern - see schemeshard__operation_backup_backup_collection.cpp:218)
+    // This ensures the index metadata schema version matches the restored impl table
+    {
+        auto indexPath = TPath::Init(indexPathId, this);
+
+        auto atTableRequest = MakeHolder<TEvSchemeShard::TEvModifySchemeTransaction>();
+        auto& atTableRecord = atTableRequest->Record;
+
+        TTxId atTableTxId = GetCachedTxId(ctx);
+        atTableRecord.SetTxId(ui64(atTableTxId));
+
+        auto& atTableTx = *atTableRecord.AddTransaction();
+        atTableTx.SetOperationType(NKikimrSchemeOp::ESchemeOpRestoreIncrementalBackupAtTable);
+        atTableTx.SetInternal(true);
+        atTableTx.SetWorkingDir(indexPath.Parent().PathString());
+
+        auto& atTableRestore = *atTableTx.MutableRestoreMultipleIncrementalBackups();
+        atTableRestore.AddSrcTablePaths(srcIndexBackupPath);
+        atTableRestore.SetDstTablePath(indexPath.PathString());
+
+        // Track this AtTable operation
+        TOperationId atTableOpId(atTableTxId, 0);
+        IncrementalRestoreOperationToState[atTableOpId] = operationId;
+        TxIdToIncrementalRestore[atTableTxId] = operationId;
+
+        if (stateIt != IncrementalRestoreStates.end()) {
+            stateIt->second.InProgressOperations.insert(atTableOpId);
+            LOG_I("Tracking index AtTable operation " << atTableOpId << " for schema version sync");
+        }
+
+        LOG_I("Sending index AtTable operation for schema version sync at: " << indexPath.PathString());
+        Send(SelfId(), atTableRequest.Release());
+    }
 }
 
 // Notification function for operation completion