[BLL/MediatR/Timeline/TimelineItem/GetById/GetTimelineItemById] Handler ignores request.Id

[InnetaSh]

GetTimelineItemByIdHandler currently ignores request.Id when retrieving a timeline item.
Because of this, the query always returns the first timeline item from the repository.
Expected behavior:
The handler should retrieve a timeline item using:
predicate: ti => ti.Id == request.Id
Tasks:
Replace incorrect predicate
Verify correct timeline item retrieval by id

User story and test case links
E.g.: "Issue #3
Test case"