fix(mcp): complete merger lease acquire + structured capability resolve (#718, #723)

Harden gitea_acquire_merger_pr_lease with required candidate_head, live-head
match, and fail-closed POST handling. Add capability-map aliases for reviewer
and merger lease acquire/adopt. Make unknown tasks return structured
unknown_task (no ValueError escape). Expand tests for roles, head scoping,
aliases, and unknown_task.

Closes #718. Addresses #723 resolver unknown_task and lease task mapping.

tests/test_resolve_task_capability.py

@@ -231,9 +231,16 @@ class TestResolveTaskCapability(unittest.TestCase):
             self.assertIn("gitea.issue.comment", res["active_profile_allowed_operations"])
 
     def test_resolve_unknown_task_fails_closed(self):
+        # #723: unknown tasks return structured unknown_task (no ValueError escape).
         with patch.dict(os.environ, self._env("author-profile")):
-            with self.assertRaises(ValueError):
-                mcp_server.gitea_resolve_task_capability(task="invalid_task_xyz", remote="prgs")
+            res = mcp_server.gitea_resolve_task_capability(
+                task="invalid_task_xyz", remote="prgs"
+            )
+            self.assertIsInstance(res, dict)
+            self.assertEqual(res.get("reason_code"), "unknown_task", res)
+            self.assertFalse(res.get("allowed_in_current_session"))
+            self.assertTrue(res.get("stop_required"))
+            self.assertIs(res.get("mutation_performed"), False)
 
     # Additional regression tests per #145 for permission boundaries and structured guidance
     def test_issue_comment_does_not_imply_close(self):
@@ -439,8 +446,11 @@ class TestResolveTaskCapability(unittest.TestCase):
             res = mcp_server.gitea_resolve_task_capability(task="close_pr", remote="prgs")
             self.assertEqual(res["required_operation_permission"], "gitea.pr.close")
             for unknown in ("close_pull_request", "close", "pr_close"):
-                with self.assertRaises(ValueError):
-                    mcp_server.gitea_resolve_task_capability(task=unknown, remote="prgs")
+                unk = mcp_server.gitea_resolve_task_capability(
+                    task=unknown, remote="prgs"
+                )
+                self.assertEqual(unk.get("reason_code"), "unknown_task", unk)
+                self.assertFalse(unk.get("allowed_in_current_session"))
 
 if __name__ == "__main__":
     unittest.main()