[BUG] eventstats/streamstats reject window functions that grammar accepts

Query Information

PPL Command/Query:

source=bounty-types | eventstats row_number() by str_field
source=bounty-types | streamstats rank() by int_field
source=bounty-types | eventstats dense_rank() by str_field

Expected Result:
Supported window functions should execute in eventstats/streamstats.

Actual Result:
Each query fails with HTTP 500 and Unexpected window function for row_number, rank, and dense_rank.

Dataset Information

Dataset/Schema Type

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

Index Mapping

{
  "mappings": {
    "properties": {
      "int_field": { "type": "integer" },
      "str_field": { "type": "keyword" }
    }
  }
}

Sample Data

[
  { "int_field": 42, "str_field": "alpha" },
  { "int_field": -1, "str_field": "alpha" },
  { "int_field": 0, "str_field": "beta" }
]

Bug Description

Issue Summary:
eventstats/streamstats rejects common window functions even though grammar accepts them.

Steps to Reproduce:

1. Create index with int_field and str_field.
2. Insert sample rows.
3. Run each query above.
4. Observe HTTP 500 Unexpected window function errors.

Impact:
Window-based analytics pipelines cannot run using these standard functions.

Start by reproducing the three PPL queries against the sample mapping and data, then trace the eventstats/streamstats handling that emits Unexpected window function. Done means row_number(), rank(), and dense_rank() execute successfully in both commands without HTTP 500 errors.