Enable BMW for single-scorer boolean queries by removing early return in scorer_union (#2915)

The early return for `scorers.len() == 1` in `scorer_union` short-circuits a single TermScorer into `SpecializedScorer::Other`, bypassing the `TermUnion` path that enables block-max WAND (BMW) in `for_each_pruning`.

This was originally addressed in PR #2898 (backed out), which added a special case in `BooleanWeight::for_each_pruning`. PR #2912 (merged as d27ca16) added a single-scorer fast path inside `block_wand` itself, but did not remove this early return — so a single SHOULD TermScorer still never reaches the BMW path.

Removing the early return lets a single TermScorer with freq reading flow through to `SpecializedScorer::TermUnion`, where `block_wand` → `block_wand_single_scorer` handles it efficiently.

Author: jamessewell

src/query/boolean_query/boolean_weight.rs

@@ -9,8 +9,8 @@ use crate::query::score_combiner::{DoNothingCombiner, ScoreCombiner};
 use crate::query::term_query::TermScorer;
 use crate::query::weight::{for_each_docset_buffered, for_each_pruning_scorer, for_each_scorer};
 use crate::query::{
-    intersect_scorers, AllScorer, BufferedUnionScorer, EmptyScorer, Exclude, Explanation,
-    Intersection, Occur, RequiredOptionalScorer, Scorer, Weight,
+    intersect_scorers, AllScorer, BufferedUnionScorer, EmptyScorer, Exclude, Explanation, Occur,
+    RequiredOptionalScorer, Scorer, Weight,
 };
 use crate::{DocId, Score};
 
@@ -50,10 +50,9 @@ where
     TScoreCombiner: ScoreCombiner,
 {
     assert!(!scorers.is_empty());
-    if scorers.len() == 1 {
+    if scorers.len() == 1 && !scorers[0].is::<TermScorer>() {
         return SpecializedScorer::Other(scorers.into_iter().next().unwrap()); //< we checked the size beforehand
     }
-
     {
         let is_all_term_queries = scorers.iter().all(|scorer| scorer.is::<TermScorer>());
         if is_all_term_queries {
@@ -67,6 +66,9 @@ where
             {
                 // Block wand is only available if we read frequencies.
                 return SpecializedScorer::TermUnion(scorers);
+            } else if scorers.len() == 1 {
+                // Single TermScorer without freq reading — unwrap directly.
+                return SpecializedScorer::Other(Box::new(scorers.into_iter().next().unwrap()));
             } else {
                 return SpecializedScorer::Other(Box::new(BufferedUnionScorer::build(
                     scorers,