Fix a bug in CleanupService._purge_sandboxes() where the internal
_sandbox_dirs_cache was never invalidated after deleting stale sandbox
directories. This caused subsequent scan() calls on the same instance to
return cached paths pointing to already-deleted sandboxes, producing
false-positive stale-item reports and potential double-delete errors.

The cache is now invalidated (set to None) whenever any sandbox directory
is successfully removed during purge, forcing subsequent scan()/purge()
calls to re-iterate /tmp for a fresh listing.

Changes:
- src/cleveragents/application/services/cleanup_service.py:
  Track whether any removals occurred in _purge_sandboxes(), reset
  _sandbox_dirs_cache = None on success (issue #7527)
- CHANGELOG.md: Added entry under [Unreleased]/Fixed for this fix
- CONTRIBUTORS.md: Added contribution attribution for HAL 9000
- features/cleanup_cache_invalidation.feature + steps: BDD test coverage

ISSUES CLOSED: #7527

@@ -1,3 +1,4 @@
+* HAL 9000 has contributed the sandbox_dirs_cache invalidation fix (PR #8257 / issue #7527): added cache invalidation in ``_purge_sandboxes()`` to prevent stale directory listings from persisting after sandbox deletion.

@@ -0,0 +13,4 @@
+  Scenario: purge without removals does not invalidate the cache
+    Given cleanup coverage has a CleanupService with default settings
+    When cleanup coverage runs purge on a service with only active plans in _purge_sandboxes (no removals)
+    Then cleanup coverage purged sandbox should keep cache unchanged

@@ -0,0 +6,4 @@
+
+from __future__ import annotations
+
+import tempfile

@@ -0,0 +22,4 @@
+# ── Helpers ──────────────────────────────────────────────────────
+
+
+def _make_settings(**overrides):

@@ -0,0 +109,4 @@
+@then("cleanup coverage purged sandbox should keep cache unchanged")
+def step_cov_cache_unchanged_no_removals(context: Context) -> None:
+    # Cache should still contain the original value since no dirs were removed
+    expected = [MagicMock()]

@@ -0,0 +110,4 @@
+def step_cov_cache_unchanged_no_removals(context: Context) -> None:
+    # Cache should still contain the original value since no dirs were removed
+    expected = [MagicMock()]
+    assert context._cov_service_after_purge._sandbox_dirs_cache is not None, (