Computing full effective mappings for data stream mapping APIs

[masseyke]

The update data stream mappings API (#130241) validates that the effective mapping after applying the user's mapping overrides is still a valid template. However, the effective mapping is generated using only the mapping overrides and the composable template's top-level mapping. It ignores any mappings provided by component templates within the composable template. This is a bug. This PR fixes that bug, and also uses the same new logic for generating the effective_mappings that are returned to the user in the get and update data stream mappings APIs.

[Copilot]

Pull Request Overview

Adds full effective mapping computation for data streams by introducing overloaded mapping methods, updating service and transport layers, and adjusting tests/specs to validate the merged mappings.

• Introduce new getEffectiveMappings overloads in DataStream requiring IndicesService.
• Update MetadataDataStreamsService and transport actions to pass an IndicesService into mapping validation.
• Refresh tests and REST-spec YAML to expect the additional timestamp and merged properties.

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
server/src/test/java/.../DataStreamTests.java	Updated test to call new getEffectiveMappings signature
server/src/main/java/.../MetadataDataStreamsService.java	Switched validateTemplate call to null mappings (should use new API)
server/src/main/java/.../DataStream.java	Added overloaded getEffectiveMappings methods with IndicesService
server/src/main/java/.../ComposableIndexTemplate.java	Made merge and convertMappingMapToXContent public
modules/data-streams/src/yamlRestTest/.../250_data_stream_mappings.yml	Updated expected property counts and new fields in spec
modules/data-streams/src/main/java/.../TransportUpdateDataStreamMappingsAction.java	Injected IndicesService, updated call to getEffectiveMappings
modules/data-streams/src/main/java/.../TransportGetDataStreamMappingsAction.java	Injected IndicesService, updated call to getEffectiveMappings
modules/data-streams/src/internalClusterTest/.../TransportUpdateDataStreamMappingsActionIT.java	Updated expected mappings to include _data_stream_timestamp and @timestamp
modules/data-streams/src/internalClusterTest/.../DataStreamIT.java	Added end-to-end testGetEffectiveMappings integration test

Comments suppressed due to low confidence (1)

server/src/test/java/org/elasticsearch/cluster/metadata/DataStreamTests.java:2692

• The call to mock(IndicesService.class) requires a static import of org.mockito.Mockito.mock. Please add import static org.mockito.Mockito.mock; to avoid a compilation error.

        IndicesService indicesService = mock(IndicesService.class);