Overview

Add a MongoExtensionStreamType enum to the public C API (api.h) that identifies the two streams a multi-stream extension source can produce.

Background

The new \$_documentResultsAndMetadata container routes documents from an extension source to two consumers via an Exchange keyed on a _streamType field. The enum defines the two stream type constants (kDocResult = 0, kMetaResult = 1) that extensions use when tagging documents and that the SDK's ExecAggStageResultsAndMetadataSource class exposes as a typed enum.

Scope of Work

* Add MongoExtensionStreamType enum to src/mongo/db/extension/public/api.h with values:
** kMongoExtensionStreamTypeDocResult = 0
** kMongoExtensionStreamTypeMetaResult = 1

Acceptance Criteria

* Enum is present in api.h and usable from both host and extension code
* Values are stable (0 and 1) to match Exchange kKeyRange boundary configuration