Cover a flat_object declared as a sub-field of an object

The field type's documentation shows this as its second example, and the tests
only had one at the top level. The leaf is then named by the whole path and the
index term carries all of it, so the lookup is the same term query -- which is
what the explain assertion checks, since the path is assembled from the column
name rather than being a single segment.

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

integ-test/src/yamlRestTest/resources/rest-api-spec/test/issues/1604.yml

@@ -194,3 +194,61 @@ teardown:
         body:
           query: "source=otel | where like(attributes.duration_ms, '1%') | fields service"
   - match: {"calcite.physical": "/\"wildcard\":\\{\"attributes.duration_ms\"/"}
+
+---
+# A flat_object can be a sub-field of an ordinary object, which the field type's documentation shows
+# as its second example. The leaf is then named by the whole path, and the index term carries all of
+# it, so the lookup is the same one.
+"a flat_object can be a sub-field of an object":
+  - skip:
+      features: [headers]
+  - do:
+      indices.create:
+        index: sub
+        body:
+          settings:
+            number_of_shards: 1
+            number_of_replicas: 0
+          mappings:
+            properties:
+              issue:
+                properties:
+                  number:
+                    type: double
+                  labels:
+                    type: flat_object
+  - do:
+      index:
+        index: sub
+        id: 1
+        refresh: true
+        body: {"issue": {"number": 123456, "labels": {"version": "2.1", "backport": ["2.0", "1.3"], "category": {"type": "API", "level": "enhancement"}}}}
+  # the sibling keeps its own mapping; only labels is flat
+  - do:
+      headers: {Content-Type: 'application/json'}
+      ppl:
+        body:
+          query: 'describe sub | fields COLUMN_NAME, TYPE_NAME'
+  - match: {"datarows": [["issue", "struct"], ["issue.labels", "flat_object"], ["issue.number", "double"]]}
+  - do:
+      headers: {Content-Type: 'application/json'}
+      ppl:
+        body:
+          query: 'source=sub | fields issue.labels.category.level, issue.labels.backport, issue.number'
+  - match: {"datarows": [["enhancement", "[\"2.0\",\"1.3\"]", 123456.0]]}
+  # the term the index holds carries the whole path, so the lookup is a term like any other
+  - do:
+      headers: {Content-Type: 'application/json'}
+      ppl:
+        body:
+          query: 'source=sub | where issue.labels.category.level = ''enhancement'' | fields issue.number'
+  - match: {"datarows": [[123456.0]]}
+  - do:
+      headers: {Content-Type: 'application/json'}
+      ppl.explain:
+        body:
+          query: "source=sub | where issue.labels.category.level = 'enhancement' | fields issue.number"
+  - match: {"calcite.physical": "/\"term\":\\{\"issue.labels.category.level\"/"}
+  - do:
+      indices.delete:
+        index: sub