Strip _doc wrapper from effective_mapping field in API responses

[augment-app-staging]

Problem

The effective_mapping field in the get data stream API response and simulate ingest API response was incorrectly including a _doc wrapper at the top level, but the spec says it should not have this wrapper. The _doc type placeholder needs to be stripped from the effective mapping output.

Solution

This PR strips the _doc wrapper from effective mappings in API responses by applying the same pattern used in Template.reduceMapping().

Changes Made:

1. SimulateIndexResponse.java - Added logic to strip _doc wrapper in innerToXContent() method
2. GetDataStreamMappingsAction.java - Added logic to strip _doc wrapper in DataStreamMappingsResponse.toXContent() method
3. UpdateDataStreamMappingsAction.java - Added logic to strip _doc wrapper in DataStreamMappingsResponse.toXContent() method
4. 80_ingest_simulate.yml - Updated YAML REST test to expect mappings without _doc wrapper

Implementation Details:

The fix checks if the mapping contains only a single _doc key and unwraps it if present:

if (mapping.size() == 1 && mapping.containsKey(MapperService.SINGLE_MAPPING_NAME)) {
    @SuppressWarnings("unchecked")
    Map<String, Object> unwrapped = (Map<String, Object>) mapping.get(MapperService.SINGLE_MAPPING_NAME);
    builder.field("effective_mapping", unwrapped);
} else {
    builder.field("effective_mapping", mapping);
}

Testing:

• ✅ All unit tests pass: SimulateIndexResponseTests, GetDataStreamMappingsActionTests, UpdateDataStreamMappingsActionResponseTests
• ✅ YAML REST test updated and passing: 80_ingest_simulate.yml

Related

• Triggered by: https://augment-wic8570.slack.com/archives/C0AG2MXKEQJ/p1772152336923009
• Triggered by: Slack-Bot-Evals