[REFACTOR] Remove obsolete tab-centric plugin assumptions

[ksamaschke]

Phase

REFACTOR

## Deliverable
Clean up obsolete tab-centric plugin assumptions while preserving route and slot contribution support.

## Dependencies
- Parent work item: #218.

## Completion evidence
- Cleanup notes for plugin runtime assumptions.

• Evidence that route and slot contribution behavior remains intact.

  Acceptance criteria

  • Plugin extension points are easier to reason about.

• The cleanup does not regress route-based plugin behavior.

  Parent: Work Item: Migrate plugin contributions from tabs to route and slot APIs #218