test: add coverage for publications listing researchers/grants population

[JanSmrcka]

Context

PR #391 moved researcher/grant fetching from N+1 service calls into CALL {} subqueries in the main Cypher query. No test asserts that GetPublications returns populated EliResearchers and Grants slices.

Risk

If someone edits the Cypher RETURN map and drops eliResearchers/grants, nothing catches it.

Acceptance

• Test that GetPublications returns non-empty EliResearchers for a publication with connected researchers
• Test that GetPublications returns non-empty Grants for a publication with connected grants
• Test that publications with no researchers/grants return empty slices (not nil)