Fix O(N) list building in TransportSearchAction.asyncSearchExecutor (elastic#112474)

This at least avoids the O(N) list building which is needlessly heavy
for large index counts. Not sure the logic makes
perfect sense in all cases, but it should remain practically unchanged
for now (except when there's more than 2 indices and they're all system
ones).

Author: original-brownbear

server/src/main/java/org/elasticsearch/action/search/TransportSearchAction.java

@@ -1285,16 +1285,27 @@ private void executeSearch(
     }
 
     Executor asyncSearchExecutor(final String[] indices) {
-        final List<String> executorsForIndices = Arrays.stream(indices).map(executorSelector::executorForSearch).toList();
-        if (executorsForIndices.size() == 1) { // all indices have same executor
-            return threadPool.executor(executorsForIndices.get(0));
+        boolean seenSystem = false;
+        boolean seenCritical = false;
+        for (String index : indices) {
+            final String executorName = executorSelector.executorForSearch(index);
+            switch (executorName) {
+                case SYSTEM_READ -> seenSystem = true;
+                case SYSTEM_CRITICAL_READ -> seenCritical = true;
+                default -> {
+                    return threadPool.executor(executorName);
+                }
+            }
         }
-        if (executorsForIndices.size() == 2
-            && executorsForIndices.contains(SYSTEM_READ)
-            && executorsForIndices.contains(SYSTEM_CRITICAL_READ)) { // mix of critical and non critical system indices
-            return threadPool.executor(SYSTEM_READ);
+        final String executor;
+        if (seenSystem == false && seenCritical) {
+            executor = SYSTEM_CRITICAL_READ;
+        } else if (seenSystem) {
+            executor = SYSTEM_READ;
+        } else {
+            executor = ThreadPool.Names.SEARCH;
         }
-        return threadPool.executor(ThreadPool.Names.SEARCH);
+        return threadPool.executor(executor);
     }
 
     static BiFunction<String, String, Transport.Connection> buildConnectionLookup(