Fix segment count for optimized agg rest test

Signed-off-by: Finn Carroll <carrofin@amazon.com>

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/360_date_histogram.yml

@@ -109,6 +109,19 @@ setup:
 
 ---
 "Date histogram aggregation w/ shared field range test":
+  - do:
+      indices.create:
+        index: dhisto-agg-w-query
+        body:
+          settings:
+            number_of_shards: 1
+            number_of_replicas: 0
+            refresh_interval: -1
+          mappings:
+            properties:
+              date:
+                type: date
+
   - do:
       bulk:
         refresh: true
@@ -127,6 +140,11 @@ setup:
           - '{"index": {}}'
           - '{"date": "2025-02-14"}'
 
+  - do:
+      indices.forcemerge:
+        index: dhisto-agg-w-query
+        max_num_segments: 1
+
   - do:
       search:
         index: dhisto-agg-w-query