test: pin proxy fixture to a single worker/acceptor to deflake Windows CI

[vdusek]

Fixes flaky test_proxy_kwarg_works_against_a_real_proxy on Windows CI (run 32528651226): an xdist worker crashed ("node down: Not properly terminated") while starting the real local proxy.

Root cause: the proxy/disabled_proxy fixtures start proxy.py's Proxy(...) without --num-workers/--num-acceptors, so it defaults both to multiprocessing.cpu_count() — spawning ~8 extra OS processes (confirmed empirically) for a single-test proxy. Under xdist's own parallel workers on a resource-constrained Windows runner, this process-spawn amplification crashed a worker. proxy.py's own test harness (proxy/testing/test_case.py) pins --num-workers 1 --num-acceptors 1 for exactly this reason.

Fix: add the same flags to both fixtures, cutting the fixture's spawned processes from ~8 to 1.

Verified 40/40 clean runs of the affected test modules under -n auto after the fix (previously confirmed 8 child processes spawned per fixture use before the fix vs. 1 after).

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.73%. Comparing base (b4c59c6) to head (249a8ce).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2188   +/-   ##
=======================================
  Coverage   93.72%   93.73%           
=======================================
  Files         181      181           
  Lines       12825    12825           
=======================================
+ Hits        12020    12021    +1     
+ Misses        805      804    -1     

Flag	Coverage Δ
unit	93.73% <ø> (+<0.01%)	⬆️

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