test: harden session-state isolation for #594 decision-lock suite

Pin per-test durable session-state so patch.dict(clear=True) cannot adopt
host ~/.cache review-decision locks, and clear the merge-test ledger in
setUp. Keeps active #332 hard-stop coverage while making TestMergePR
deterministic without manual host cache cleanup.

Aligned with #590 / PR #592 isolation approach.

tests/test_mcp_server.py

@@ -774,6 +774,9 @@ class TestMergePR(unittest.TestCase):
     def setUp(self):
         import reviewer_pr_lease
 
+        # Clean ledger each merge test so residual/host durable #332 state
+        # cannot short-circuit eligibility gates (#590 / #594).
+        mcp_server._save_review_decision_lock(None)
         mcp_server.gitea_load_review_workflow()
         self._lease_patch = _install_owned_reviewer_lease(8)
         self._lease_patch.start()