[#23257] YSQL: Change conflict error string for RC transactions

Summary:
Currently DocDB throws the same error string to YSQL for the following 2
situations:
(1) Transaction conflicts with a higher priority transaction.
(2) Transaction conflicts with a transaction having the same priority.

The error string is "conflicts with higher priority transaction". This
diff separates the two cases and throws "conflicts with same priority
transaction" for case (2).
Jira: DB-12186

Test Plan: Jenkins

Reviewers: patnaik.balivada

Reviewed By: patnaik.balivada

Subscribers: bkolagani, ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D36750

src/yb/docdb/conflict_resolution.cc

@@ -1045,7 +1045,9 @@ class ConflictResolverContextBase : public ConflictResolverContext {
       //   2. a kConflict is raised even if their_priority equals our_priority.
       if (our_priority <= their_priority) {
         return MakeConflictStatus(
-            our_transaction_id, transaction.id, "higher priority", GetTabletMetrics());
+            our_transaction_id, transaction.id,
+            our_priority == their_priority ? "same priority" : "higher priority",
+            GetTabletMetrics());
       }
     }
     fetched_metadata_for_transactions_ = true;