Fix lock_store crash when redis package is not installed (#4943)

* Fix lock_store crash when redis package is not installed

`REDIS_URL` being set was enough to trigger a Redis lock, which would
raise `ImportError` if the `redis` package wasn't available. Added
`_redis_available()` to guard on both the env var and the import.

* Simplify tests

* Simplify tests #2

lib/crewai/tests/utilities/test_lock_store.py

@@ -0,0 +1,70 @@
+"""Tests for lock_store.
+
+We verify our own logic: the _redis_available guard and which portalocker
+backend is selected. We trust portalocker to handle actual locking mechanics.
+"""
+
+from __future__ import annotations
+
+import sys
+from unittest import mock
+
+import pytest
+
+import crewai.utilities.lock_store as lock_store
+from crewai.utilities.lock_store import lock
+
+
+@pytest.fixture(autouse=True)
+def no_redis_url(monkeypatch):
+    monkeypatch.setattr(lock_store, "_REDIS_URL", None)
+
+
+# ---------------------------------------------------------------------------
+# _redis_available
+# ---------------------------------------------------------------------------
+
+
+def test_redis_not_available_without_url():
+    assert lock_store._redis_available() is False
+
+
+def test_redis_not_available_when_package_missing(monkeypatch):
+    monkeypatch.setattr(lock_store, "_REDIS_URL", "redis://localhost:6379")
+    monkeypatch.setitem(sys.modules, "redis", None)  # None → ImportError on import
+    assert lock_store._redis_available() is False
+
+
+def test_redis_available_with_url_and_package(monkeypatch):
+    monkeypatch.setattr(lock_store, "_REDIS_URL", "redis://localhost:6379")
+    monkeypatch.setitem(sys.modules, "redis", mock.MagicMock())
+    assert lock_store._redis_available() is True
+
+
+# ---------------------------------------------------------------------------
+# lock strategy selection
+# ---------------------------------------------------------------------------
+
+
+def test_uses_file_lock_when_redis_unavailable():
+    with mock.patch("portalocker.Lock") as mock_lock:
+        with lock("file_test"):
+            pass
+
+    mock_lock.assert_called_once()
+    assert "crewai:" in mock_lock.call_args.args[0]
+
+
+def test_uses_redis_lock_when_redis_available(monkeypatch):
+    fake_conn = mock.MagicMock()
+    monkeypatch.setattr(lock_store, "_redis_available", mock.Mock(return_value=True))
+    monkeypatch.setattr(lock_store, "_redis_connection", mock.Mock(return_value=fake_conn))
+
+    with mock.patch("portalocker.RedisLock") as mock_redis_lock:
+        with lock("redis_test"):
+            pass
+
+    mock_redis_lock.assert_called_once()
+    kwargs = mock_redis_lock.call_args.kwargs
+    assert kwargs["channel"].startswith("crewai:")
+    assert kwargs["connection"] is fake_conn