Replace OS-scheduler-dependent elapsed-time ceilings with deterministic
concurrency proofs in three tests:
- test_context_refs_concurrent: asyncio.Barrier rendezvous — all 3 URL
fetches must be in flight simultaneously before any returns.
- test_memory_boundary_commit: positive non-blocking witness — the
provider call list must still be empty when the async commit returns.
- test_mem0_v3 slow-prefetch: threading.Event park/release — prefetch
must return while the backend search is still parked.
The tests/tools/test_mcp_tool.py hunk from the original PR is dropped:
main no longer carries the 'elapsed < 2.5' assertion it targeted
(superseded by a delay-relative bound).
Salvaged from #71913.
9c28600e77