feat: enforce session hard-stop after terminal review mutation (#332)

Extends the single-terminal-decision machinery (#211) to session-level
hard-stop semantics:

- terminal_review_hard_stop_reasons: after a live terminal verdict, only
  the merge sequence for the same approved PR may continue; REQUEST_CHANGES
  stops the run; operator-approved corrections re-open the review path
  only, never a cross-PR merge.
- gitea_merge_pr: new Gate 2b consults the hard-stop before any API call,
  so a run can never verdict one PR and merge another.
- gitea_mark_final_review_decision: hard-stop check with explicit stop
  guidance naming the consumed terminal mutation; plus duplicate
  REQUEST_CHANGES suppression — an unresolved request-changes at the
  current head SHA refuses a second request-changes (fail closed when
  feedback cannot be verified).
- tests/test_terminal_review_hard_stop.py: 13 tests covering the
  acceptance criteria; existing TestSubmitPrReview setups stub the new
  feedback fetch.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

tests/test_mcp_server.py

@@ -47,6 +47,21 @@ import mcp_server
 
 FAKE_AUTH = "Basic dGVzdDp0ZXN0"
 
+_NO_BLOCKER_FEEDBACK = {
+    "success": True,
+    "has_blocking_change_requests": False,
+    "review_feedback_stale": False,
+}
+
+
+def _mark_request_changes_ready(pr_number=8, **kwargs):
+    """Mark a request_changes decision ready with the #332 duplicate-
+    suppression feedback fetch stubbed to 'no existing blocker'."""
+    with patch("mcp_server.gitea_get_pr_review_feedback",
+               return_value=dict(_NO_BLOCKER_FEEDBACK)):
+        return gitea_mark_final_review_decision(
+            pr_number, "request_changes", **kwargs)
+
 
 def _formal_review(reviewer, verdict, sha="abc123", review_id=1):
     return {
@@ -1775,7 +1790,7 @@ class TestSubmitPrReview(unittest.TestCase):
     @patch("mcp_server.api_request")
     @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)
     def test_request_changes_succeeds_when_eligible(self, _auth, mock_api):
-        gitea_mark_final_review_decision(8, "request_changes", remote="prgs")
+        _mark_request_changes_ready(remote="prgs")
         mock_api.side_effect = [
             {"login": "reviewer-bot"}, self._pr("author-bot"),
             {"id": 9, "state": "REQUEST_CHANGES"},
@@ -1798,7 +1813,7 @@ class TestSubmitPrReview(unittest.TestCase):
         self.assertEqual(post_calls[0].args[3]["event"], "REQUEST_CHANGES")
 
     def test_request_changes_blocked_without_eligibility(self):
-        gitea_mark_final_review_decision(8, "request_changes", remote="prgs")
+        _mark_request_changes_ready(remote="prgs")
         with patch("mcp_server.get_auth_header", return_value=FAKE_AUTH) as _a, \
                 patch("mcp_server.api_request") as mock_api:
             mock_api.side_effect = [{"login": "reviewer-bot"}, self._pr("author-bot")]
@@ -2080,7 +2095,7 @@ class TestSubmitPrReview(unittest.TestCase):
                 reason="operator approved correcting mistaken approve",
                 operator_authorized=True,
             )
-            gitea_mark_final_review_decision(8, "request_changes", remote="prgs")
+            _mark_request_changes_ready(remote="prgs")
             second = gitea_submit_pr_review(
                 pr_number=8, action="request_changes", remote="prgs",
                 final_review_decision_ready=True,