Add an additional tiebreaker to RRF (elastic#101847)

This change adds an additional tiebreaker for RRF where when two documents have the same RRF 
"score" such as identical ranks of (3,4) and (4,3) or (1,-) and (-,1), etc. the ordering will fallback to the 
highest score from query 1 then query 2, and so on. If all scores are equal then the tiebreaker will be 
shard index followed by doc id, but these are not necessarily stable. This should resolve most of the 
stability issues outlined as part of (elastic#101232).

Closes elastic#101232

docs/changelog/101847.yaml

@@ -0,0 +1,6 @@
+pr: 101847
+summary: Add an additional tiebreaker to RRF
+area: Ranking
+type: bug
+issues:
+ - 101232

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRankCoordinatorContext.java

@@ -127,12 +127,25 @@ protected boolean lessThan(RRFRankDoc a, RRFRankDoc b) {
             }
         }
 
-        // sort the results based on rrf score, tiebreaker based on smaller shard then smaller doc id
+        // sort the results based on rrf score, tiebreaker based on
+        // larger individual query score from 1 to n, smaller shard then smaller doc id
         RRFRankDoc[] sortedResults = results.values().toArray(RRFRankDoc[]::new);
         Arrays.sort(sortedResults, (RRFRankDoc rrf1, RRFRankDoc rrf2) -> {
             if (rrf1.score != rrf2.score) {
                 return rrf1.score < rrf2.score ? 1 : -1;
             }
+            assert rrf1.positions.length == rrf2.positions.length;
+            for (int qi = 0; qi < rrf1.positions.length; ++qi) {
+                if (rrf1.positions[qi] != NO_RANK && rrf2.positions[qi] != NO_RANK) {
+                    if (rrf1.scores[qi] != rrf2.scores[qi]) {
+                        return rrf1.scores[qi] < rrf2.scores[qi] ? 1 : -1;
+                    }
+                } else if (rrf1.positions[qi] != NO_RANK) {
+                    return -1;
+                } else if (rrf2.positions[qi] != NO_RANK) {
+                    return 1;
+                }
+            }
             if (rrf1.shardIndex != rrf2.shardIndex) {
                 return rrf1.shardIndex < rrf2.shardIndex ? -1 : 1;
             }

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRankShardContext.java

@@ -17,6 +17,8 @@
 import java.util.List;
 import java.util.Map;
 
+import static org.elasticsearch.search.rank.RankDoc.NO_RANK;
+
 /**
  * Executes queries and generates results on the shard for RRF.
  */
@@ -74,6 +76,18 @@ public RRFRankShardResult combine(List<TopDocs> rankResults) {
             if (rrf1.score != rrf2.score) {
                 return rrf1.score < rrf2.score ? 1 : -1;
             }
+            assert rrf1.positions.length == rrf2.positions.length;
+            for (int qi = 0; qi < rrf1.positions.length; ++qi) {
+                if (rrf1.positions[qi] != NO_RANK && rrf2.positions[qi] != NO_RANK) {
+                    if (rrf1.scores[qi] != rrf2.scores[qi]) {
+                        return rrf1.scores[qi] < rrf2.scores[qi] ? 1 : -1;
+                    }
+                } else if (rrf1.positions[qi] != NO_RANK) {
+                    return -1;
+                } else if (rrf2.positions[qi] != NO_RANK) {
+                    return 1;
+                }
+            }
             return rrf1.doc < rrf2.doc ? -1 : 1;
         });
         // trim the results to window size