fix(tests): prevent xdist worker OOM from leaked tarpit asyncio task

asyncio_default_fixture_loop_scope was 'module', so all async tests in
a module share one event loop. test_lifespan_startup_and_shutdown patched
log_ingestion_worker/log_collector_worker/attacker_profile_worker but not
tarpit_watcher_worker — the real while-True coroutine was created as an
asyncio task on the shared loop and never cancelled. The xdist worker ran
for 4+ hours (confirmed via py-spy + etime=04:48) consuming 15+ GB before
OOM-kill.

Fixes:
- Patch tarpit_watcher_worker in both TestLifespan tests
- Change asyncio_default_fixture_loop_scope to 'function' so each test
  gets its own loop; tasks cannot outlive their test
- Add loop_scope='module' to precision_engine which legitimately needs
  a module-scoped event loop

pyproject.toml

@@ -116,7 +116,7 @@ decnet = "decnet.cli:app"
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 asyncio_debug = "true"
-asyncio_default_fixture_loop_scope = "module"
+asyncio_default_fixture_loop_scope = "function"
 addopts = "-v -q -x -n 4 --dist load"
 norecursedirs = [
     "tests/live",