E2E: Hub-and-Spoke topology across 3 clusters (Partial Mesh MVP)

## Summary
Add an end-to-end test validating Hub-and-Spoke topology behavior.

## Test scenario (MVP)
- Setup: 3 worker clusters A, B, C
- Create slice with:
- hubs: [A]
- spokes: [B, C] (or default)
- Verify:
- Intended connections exist: A<->B and A<->C
- Non-intended connection does not exist: B<->C
- Update slice hubs to [B]
- Verify rewire:
- B<->A and B<->C exist
- A<->C removed if it was only hub-driven (depending on implementation specifics)
- No B<->C direct edge unless hub requires it

## Acceptance Criteria
- Test is automated and reproducible (CI preferred; otherwise documented local harness)
- Fails reliably when topology is incorrect
- Covers create + update paths

## Tasks
- [ ] Build/extend test harness
- [ ] Implement assertions for intended/non-intended links
- [ ] Add update test (hub switch)