Expression script engine cleanups around _value usage (elastic#99706)

We have found some inconsistencies as part of elastic#99667, around the current usages of ReplaceableConstDoubleValueSource in ExpressionsScriptEngine. It looks like _value is exposed to the bindings of score scripts, but setValue is never called hence it will always be 0. That can be replaced with a constant double values source, but the next question is whether it even needs to be added to the bindings then.

Another cleanup discussed in elastic#99667 is throwing UnsupportedOperationException from ReplaceableConstDoubleValueSource#explain as it should never be called. Implementing the method means we need to test it which makes little sense if the method is never called in production code.

modules/lang-expression/src/main/java/org/elasticsearch/script/expression/ExpressionScriptEngine.java

@@ -371,16 +371,14 @@ private static ScoreScript.LeafFactory newScoreScript(Expression expr, SearchLoo
         // NOTE: if we need to do anything complicated with bindings in the future, we can just extend Bindings,
         // instead of complicating SimpleBindings (which should stay simple)
         SimpleBindings bindings = new SimpleBindings();
-        ReplaceableConstDoubleValueSource specialValue = null;
         boolean needsScores = false;
         for (String variable : expr.variables) {
             try {
                 if (variable.equals("_score")) {
                     bindings.add("_score", DoubleValuesSource.SCORES);
                     needsScores = true;
                 } else if (variable.equals("_value")) {
-                    specialValue = new ReplaceableConstDoubleValueSource();
-                    bindings.add("_value", specialValue);
+                    bindings.add("_value", DoubleValuesSource.constant(0));
                     // noop: _value is special for aggregations, and is handled in ExpressionScriptBindings
                     // TODO: if some uses it in a scoring expression, they will get a nasty failure when evaluating...need a
                     // way to know this is for aggregations and so _value is ok to have...

modules/lang-expression/src/main/java/org/elasticsearch/script/expression/ReplaceableConstDoubleValueSource.java

@@ -38,13 +38,8 @@ public boolean needsScores() {
     }
 
     @Override
-    public Explanation explain(LeafReaderContext ctx, int docId, Explanation scoreExplanation) throws IOException {
-        // TODO where is this explain called? I bet it's never tested, and probably never called.
-        ReplaceableConstDoubleValues fv = specialValues.get(ctx);
-        if (fv.advanceExact(docId)) {
-            return Explanation.match((float) fv.doubleValue(), "ReplaceableConstDoubleValues");
-        }
-        return Explanation.noMatch("ReplaceableConstDoubleValues");
+    public Explanation explain(LeafReaderContext ctx, int docId, Explanation scoreExplanation) {
+        throw new UnsupportedOperationException("explain is not supported for _value and should never be called");
     }
 
     @Override