[8.13] ESQL: Fix count pushdown for unmapped fields (#106690) (#106710)

Backport #106690 but avoid introducing the same optimization regression by also including the fix from #106720.

Author: alex-spies

x-pack/plugin/esql/qa/testFixtures/src/main/resources/stats.csv-spec

@@ -1217,3 +1217,12 @@ FROM airports
 c:l
 891
 ;
+
+countMV#[skip:-8.13.0,reason:fixed in 8.13.1]
+FROM employees
+| STATS vals = COUNT(salary_change.int)
+;
+
+vals:l
+183
+;

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

@@ -195,11 +195,13 @@ public boolean isSingleValue(String field) {
             if (exists(field) == false) {
                 stat.singleValue = true;
             } else {
-                var sv = new boolean[] { true };
+                // fields are MV per default
+                var sv = new boolean[] { false };
                 for (SearchContext context : contexts) {
                     var sec = context.getSearchExecutionContext();
-                    MappedFieldType mappedType = sec.isFieldMapped(field) ? null : sec.getFieldType(field);
+                    MappedFieldType mappedType = sec.isFieldMapped(field) ? sec.getFieldType(field) : null;
                     if (mappedType != null) {
+                        sv[0] = true;
                         doWithContexts(r -> {
                             sv[0] &= detectSingleValue(r, mappedType, field);
                             return sv[0];