tests: pin ink engine in _make_tui_argv npm-bootstrap tests (post-merge semantic fix)

Main's rewritten test_tui_npm_install.py tests call _make_tui_argv expecting
the Ink/npm flow unconditionally; with the dual-engine dispatch merged in,
_resolve_tui_engine() auto-selects opentui whenever ui-opentui/dist is built
in the repo, routing the call away from the path under test (first subprocess
became 'node --version' instead of 'npm run build'). Pin the engine to ink
via an autouse fixture, mirroring the existing pinning precedent in
test_tui_resume_flow.py.

tests/test_plugin_utils.py

@@ -0,0 +1,159 @@
+"""Tests for plugins/plugin_utils.py — thread-safe lazy singleton helpers.
+
+These exercise the actual concurrency guarantee with real threads (not mocks):
+a barrier releases N threads simultaneously into the accessor, and we assert
+the factory ran exactly once.
+"""
+
+import threading
+
+import pytest
+
+from plugins.plugin_utils import SingletonSlot, lazy_singleton
+
+
+# --- lazy_singleton -------------------------------------------------------
+
+
+def test_lazy_singleton_builds_once_and_returns_same_instance():
+    calls = []
+
+    @lazy_singleton
+    def get():
+        calls.append(1)
+        return object()
+
+    a = get()
+    b = get()
+    assert a is b
+    assert len(calls) == 1
+
+
+def test_lazy_singleton_reset_rebuilds():
+    counter = {"n": 0}
+
+    @lazy_singleton
+    def get():
+        counter["n"] += 1
+        return counter["n"]
+
+    assert get() == 1
+    assert get() == 1
+    get.reset()
+    assert get() == 2
+
+
+def test_lazy_singleton_factory_exception_not_cached():
+    state = {"fail": True}
+
+    @lazy_singleton
+    def get():
+        if state["fail"]:
+            raise RuntimeError("boom")
+        return "ok"
+
+    with pytest.raises(RuntimeError):
+        get()
+    # First call raised → nothing cached → retry succeeds once we stop failing.
+    state["fail"] = False
+    assert get() == "ok"
+
+
+def test_lazy_singleton_concurrent_first_call_builds_once():
+    build_count = {"n": 0}
+    build_lock = threading.Lock()
+    barrier = threading.Barrier(16)
+    results = []
+    results_lock = threading.Lock()
+
+    @lazy_singleton
+    def get():
+        # Count builds under a lock so the assertion is exact even if the
+        # double-checked lock had a bug and let two through.
+        with build_lock:
+            build_count["n"] += 1
+        # Simulate an expensive build so threads genuinely overlap.
+        import time
+        time.sleep(0.01)
+        return object()
+
+    def worker():
+        barrier.wait()  # release all threads at once
+        obj = get()
+        with results_lock:
+            results.append(obj)
+
+    threads = [threading.Thread(target=worker) for _ in range(16)]
+    for t in threads:
+        t.start()
+    for t in threads:
+        t.join()
+
+    assert build_count["n"] == 1, "factory must run exactly once under race"
+    assert len(results) == 16
+    assert all(r is results[0] for r in results), "all callers share one instance"
+
+
+# --- SingletonSlot --------------------------------------------------------
+
+
+def test_slot_caches_first_value():
+    slot: SingletonSlot = SingletonSlot()
+    assert slot.peek() is None
+    v1 = slot.get(lambda: "first")
+    assert slot.peek() == "first"
+    # Subsequent factory is ignored — first value wins.
+    v2 = slot.get(lambda: "second")
+    assert v1 == v2 == "first"
+
+
+def test_slot_reset():
+    slot: SingletonSlot = SingletonSlot()
+    slot.get(lambda: "a")
+    slot.reset()
+    assert slot.peek() is None
+    assert slot.get(lambda: "b") == "b"
+
+
+def test_slot_factory_exception_not_cached():
+    slot: SingletonSlot = SingletonSlot()
+
+    def boom():
+        raise ValueError("nope")
+
+    with pytest.raises(ValueError):
+        slot.get(boom)
+    assert slot.peek() is None
+    assert slot.get(lambda: "recovered") == "recovered"
+
+
+def test_slot_concurrent_first_call_builds_once():
+    build_count = {"n": 0}
+    build_lock = threading.Lock()
+    barrier = threading.Barrier(16)
+    slot: SingletonSlot = SingletonSlot()
+    results = []
+    results_lock = threading.Lock()
+
+    def factory():
+        with build_lock:
+            build_count["n"] += 1
+        import time
+        time.sleep(0.01)
+        return object()
+
+    def worker():
+        barrier.wait()
+        obj = slot.get(factory)
+        with results_lock:
+            results.append(obj)
+
+    threads = [threading.Thread(target=worker) for _ in range(16)]
+    for t in threads:
+        t.start()
+    for t in threads:
+        t.join()
+
+    assert build_count["n"] == 1
+    assert len(results) == 16
+    assert all(r is results[0] for r in results)