Replace nest-asyncio with a dedicated background event loop

[dotsdl]

Background

The synchronous client drives async methods through BaseClient._run_async. Those methods do real concurrent I/O — batched asyncio.gather fan-out in _batched_attribute_getter/_setter, and concurrent ProtocolDAGResult retrieval — so the async stack earns its keep and isn't going anywhere.

The problem was only ever how sync called into async: _run_async used nest_asyncio to permit re-entrant run_until_complete when a loop is already running (e.g. Jupyter). nest_asyncio achieves this by monkeypatching asyncio's private internals — fragile across CPython releases and widely considered bad practice. It was also an undeclared-in-test-env dependency, which recently turned CI red across main and every open PR (ModuleNotFoundError: No module named 'nest_asyncio').

Change

Run a single persistent event loop in a dedicated daemon thread and submit coroutines with asyncio.run_coroutine_threadsafe(coro, loop).result(). The calling thread blocks on the result while the loop thread does the work.

• Concurrency preserved — all gather/as_completed fan-out runs inside the loop thread, unchanged.
• Re-entrancy without monkeypatching — a caller already inside a running loop (Jupyter) simply waits on another thread; no patching of asyncio internals.
• Stable cache binding — alru_cache binds to the loop it first runs on; a single process-wide loop keeps that invariant solid (this is what the old per-class _shared_event_loop dance was straining toward).
• Loop is created lazily under a threading.Lock so concurrent callers can't start competing loops.
• nest-asyncio dropped from alchemiscale-client.yml — no longer used anywhere.

Verification

Behavioral smoke test of the new machinery passes, including the key cases: loop reuse across calls, concurrent gather inside a submission, and invocation from within an already-running event loop (the scenario nest_asyncio existed for). Existing client integration tests exercise the real paths.

Relationship to #505

This supersedes #505 (which added nest-asyncio to the test env as an immediate CI unblock). Once this merges, nest-asyncio is no longer needed at all and #505 can be closed. Merging #505 first is harmless — this PR then removes the now-unused dependency.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.56%. Comparing base (4ce245d) to head (0425e28).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
alchemiscale/base/client.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
+ Coverage   78.91%   79.56%   +0.65%     
==========================================
  Files          29       29              
  Lines        4808     4830      +22     
==========================================
+ Hits         3794     3843      +49     
+ Misses       1014      987      -27     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.