fix(cleanup): isolate tempdir in cache invalidation test to prevent cross-test pollution

The 'scan after purge returns empty stale items' scenario was failing in CI
because _get_sandbox_dirs() re-reads tempfile.gettempdir() after cache
invalidation, which returns the nox session temp dir containing sandbox
directories from other concurrently-running tests.

Fix: patch tempfile.gettempdir to return the test's isolated temp directory
throughout the entire scan -> purge -> scan cycle, preventing unrelated
sandbox directories from appearing in the second scan's results.

features/steps/cleanup_service_cache_invalidation_steps.py

@@ -117,16 +117,28 @@ def step_cache_inv_scan_purge_scan(context: Context) -> None:
     svc = context.cache_inv_service
     real_dirs = context.cache_inv_real_dirs
 
-    # First scan: inject the real dirs into the cache
-    svc._sandbox_dirs_cache = list(real_dirs)
-    context.cache_inv_first_scan = svc.scan()
+    tmp = context.cache_inv_tmp
 
-    # Purge: actually delete the directories; cache should be invalidated
-    context.cache_inv_purge_report = svc.purge()
+    # Patch tempfile.gettempdir to return our isolated temp dir throughout
+    # the entire scan -> purge -> scan cycle.  This prevents _get_sandbox_dirs()
+    # from picking up unrelated sandbox directories created by other tests
+    # running in the same nox session.
+    with patch(
+        "cleveragents.application.services.cleanup_service.tempfile"
+    ) as mock_tmp:
+        mock_tmp.gettempdir.return_value = str(tmp)
 
-    # Second scan: cache was invalidated, so _get_sandbox_dirs re-reads /tmp
-    # The real dirs are now gone, so the scan should find nothing stale.
-    context.cache_inv_second_scan = svc.scan()
+        # First scan: inject the real dirs into the cache
+        svc._sandbox_dirs_cache = list(real_dirs)
+        context.cache_inv_first_scan = svc.scan()
+
+        # Purge: actually delete the directories; cache should be invalidated
+        context.cache_inv_purge_report = svc.purge()
+
+        # Second scan: cache was invalidated, so _get_sandbox_dirs re-reads
+        # our isolated temp dir.  The real dirs are now gone, so the scan
+        # should find nothing stale.
+        context.cache_inv_second_scan = svc.scan()
 
 
 @when("cache invalidation calls purge then scan")