[BUG] Cannot pushdown a nested root as group-by key

Query Information

PPL Command/Query:

source=test | stats count() by rec
source=test | top rec
source=test | dedup rec

Dataset Information

Dataset/Schema Type

• OpenTelemetry (OTEL)
• Simple Schema for Observability (SS4O)
• Open Cybersecurity Schema Framework (OCSF)
• Custom (details below)

Index Mapping

{
  "mappings": {
    "properties": {
      "rec": {
        "type": "nested",
        "properties": {
          "inner": {
            "type": "integer"
          }
        }
      }
    }
  }
}

Sample Data

{"rec":{"inner": 100}}
{"rec":{"inner": 101}}
{"rec":{"inner": 101}}

Bug Description

Issue Summary:
The queries above will try to trigger an aggregation pushdown which the group-by key (bucket) is rec. Since rec is a nested root, OpenSeach DSL returns null if the bucket fields is a nested field. So it will fallback to execute in Calcite EnumerableAggregate which may have performance issue.

Start with the three PPL queries and the nested mapping shown in the issue, then trace how aggregation pushdown handles the group-by key rec. Verify behavior against the OpenSearch DSL result and the Calcite EnumerableAggregate fallback. Done means nested-root grouping no longer produces an unusable pushdown result and the affected queries behave correctly.