test(core): Add comprehensive tests for SparseVector neighbor exchange

- Add tests for bidirectional exchange between neighbors
- Test ring topology with left/right neighbor exchanges
- Test data-only runtime phase with multiple steps
- Test large data exchange (10K elements) and empty vectors
- Test multiple simultaneous neighbors and 2D grid patterns
- Test different MPI tags for simultaneous exchanges
- Test gather-scatter round-trip via exchange operations
- Require MPI (2-4 processes depending on test case)

Author: ahojukka5