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(superseded by a delay-relative bound).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.
Multiple @-references in one message (esp. @url: refs, each a full
web_extract round-trip) were expanded in a serial `for ref in refs: await`
loop. Switch to asyncio.gather over the independent _expand_reference calls,
reassembling warnings/blocks in original positional order so output is
byte-identical to the serial path; the token-budget check is unchanged.
Generic + provider-agnostic: helps every web backend equally (exa/tavily/
firecrawl/parallel) since it's above the provider layer. RED/GREEN test:
3 url refs @ 0.2s each = 0.60s serial -> ~0.20s concurrent.
