Depends on: T433336: 👓 Extend the build:spec step to aggregate OpenAPI fragments from Wikibase extensions
Author the OpenAPI spec fragment for the existing WikibaseLexeme REST endpoints, starting with GET /entities/lexemes/{lexeme_id}.
Acceptance Criteria:
- A spec fragment documents each existing Lexeme endpoint: paths, parameters, request and response schemas, and status codes.
- Statement definitions are reused from the Wikibase CRUD domain rather than redefined.
- The fragment is picked up by build:spec and appears in the compiled openapi.json.