fix: isolate review-mutation ledger from host session-state (#590)

Tests that call patch.dict(os.environ, clear=True) dropped
GITEA_MCP_SESSION_STATE_DIR and re-adopted the operator host cache
under ~/.cache/gitea-tools/session-state. A residual terminal
request_changes mutation then tripped the #332 hard-stop before
test_unknown_profile_blocks could assert the empty-profile gate.

- Pin mcp_session_state.default_state_dir / DEFAULT_STATE_DIR to a
  per-test temp dir in conftest (still honors an explicit env override)
- Clear the decision lock at the start of each TestMergePR test
- Add regression coverage for env-clear isolation

Closes #590

tests/conftest.py

@@ -20,9 +20,11 @@ def _reset_mutation_authority(monkeypatch):
 
     Session-state isolation (#559 / #590 / #594): many tests use
     ``patch.dict(os.environ, ..., clear=True)``, which drops
-    ``GITEA_MCP_SESSION_STATE_DIR``. Pin ``default_state_dir`` /
-    ``DEFAULT_STATE_DIR`` to a per-test temp dir so durable load/save never
-    touches host state even after env clears.
+    ``GITEA_MCP_SESSION_STATE_DIR``. Relying only on the env var therefore
+    re-exposes the operator host cache
+    (``~/.cache/gitea-tools/session-state``) and its review-mutation ledger.
+    Pin ``default_state_dir`` / ``DEFAULT_STATE_DIR`` to a per-test temp dir
+    so durable load/save never touches host state even after env clears.
     """
     monkeypatch.delenv("GITEA_SESSION_PROFILE_LOCK", raising=False)
     # Isolate durable session-state files so tests never share host cache (#559).
@@ -36,6 +38,9 @@ def _reset_mutation_authority(monkeypatch):
     except Exception:
         mcp_session_state = None
     if mcp_session_state is not None:
+        # Survive patch.dict(..., clear=True) that drops the env var (#590).
+        # Still honor an explicit GITEA_MCP_SESSION_STATE_DIR when tests set
+        # their own temp dir (see tests/test_mcp_session_state.py).
         monkeypatch.setattr(
             mcp_session_state, "DEFAULT_STATE_DIR", state_dir, raising=False
         )