ESQL: Fix constant keyword optimization (#129278)

Fixes the ESQL's detection of `constant_keyword` fields. We unplugged it
when we changed a function signature because we didn't have an
`@Override` annotation. This plugs it back in and adds it to the
integration tests we use for pushing queries to lucene. When you do
`| WHERE constant_keyword_field == "itsvalue"` then the whole is removed
from the query plan because *all* documents are equal.

Author: nik9000

docs/changelog/129278.yaml

@@ -0,0 +1,5 @@
+pr: 129278
+summary: Fix constant keyword optimization
+area: ES|QL
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/common/lucene/search/CaseInsensitiveTermQuery.java

@@ -28,6 +28,15 @@ public CaseInsensitiveTermQuery(Term term) {
 
     @Override
     public String toString(String field) {
-        return this.getClass().getSimpleName() + "{" + field + ":" + term.text() + "}";
+        StringBuilder buffer = new StringBuilder();
+        buffer.append(getClass().getSimpleName());
+        buffer.append('{');
+        if (term.field().equals(field) == false) {
+            buffer.append(term.field());
+            buffer.append(':');
+        }
+        buffer.append(term.text());
+        buffer.append('}');
+        return buffer.toString();
     }
 }

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/LocalSourceOperator.java

@@ -82,7 +82,10 @@ public Page getOutput() {
     }
 
     @Override
-    public void close() {
+    public void close() {}
 
+    @Override
+    public String toString() {
+        return "LocalSourceOperator";
     }
 }