Harden assertions when checking target indices (elastic#105001)

We should have checked and failed if there is an inconsistent pair of
data node plan and target indices. This PR strengthens these checks and
adds assertions to fail hard in tests.

Relates elastic#10480

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plugin/ComputeService.java

@@ -149,8 +149,13 @@ public void execute(
         var concreteIndices = PlannerUtils.planConcreteIndices(physicalPlan);
 
         QueryPragmas queryPragmas = configuration.pragmas();
-
-        if (concreteIndices.isEmpty()) {
+        if (dataNodePlan == null) {
+            if (concreteIndices.isEmpty()) {
+                String error = "expected no concrete indices without data node plan; got " + concreteIndices;
+                assert false : error;
+                listener.onFailure(new IllegalStateException(error));
+                return;
+            }
             var computeContext = new ComputeContext(sessionId, List.of(), configuration, null, null);
             runCompute(
                 rootTask,
@@ -160,6 +165,12 @@ public void execute(
             );
             return;
         }
+        if (concreteIndices.isEmpty()) {
+            var error = "expected concrete indices with data node plan but got empty; data node plan " + dataNodePlan;
+            assert false : error;
+            listener.onFailure(new IllegalStateException(error));
+            return;
+        }
         QueryBuilder requestFilter = PlannerUtils.requestFilter(dataNodePlan);
 
         LOGGER.debug("Sending data node plan\n{}\n with filter [{}]", dataNodePlan, requestFilter);