feat(tests): let managers escalate and directly resolve disputed tests

Request Discussion only ever nudged the peer lead, and the one-time
manager notification when a dispute starts had no real follow-up
action attached to it — a manager could be told a dispute existed but
had no way to actually do anything about it. Adds:

- POST /tests/{id}/escalate-to-manager — either lead can explicitly
  ask managers to step in, distinct from the passive initial notice.
- POST /tests/{id}/manager-resolve-dispute — a manager decides the
  final outcome (validated/rejected) directly, bypassing both leads'
  votes entirely, reusing the same dual-validation event dispatch so
  Jira/notifications behave like any other resolution.

Both leads are notified of the manager's final call, win or lose.

backend/app/domain/test_entity.py

@@ -672,6 +672,27 @@ class TestEntity:
 
         self._events.append(DomainEvent("dispute_resolved_to_rework", {"target": target}))
 
+    def resolve_dispute_by_manager(self, outcome: str) -> None:
+        """A manager makes the final call on a disputed test, ending the standoff.
+
+        Unlike ``resolve_dispute_reject`` (a lead flipping their own vote),
+        this bypasses both leads entirely — the manager's decision is final
+        and terminal, going straight to ``validated`` or the normal
+        ``rejected`` dead-end (which any lead can later reopen via the
+        standard reopen-to-draft flow, same as any other rejection).
+
+        Args:
+            outcome (str): ``"validated"`` or ``"rejected"``.
+        """
+        if outcome not in ("validated", "rejected"):
+            raise InvalidOperationError("outcome must be 'validated' or 'rejected'")
+
+        target_state = TestState.validated if outcome == "validated" else TestState.rejected
+        self._transition(target_state)
+        self._events.append(
+            DomainEvent("dual_validation_approved" if outcome == "validated" else "dual_validation_rejected")
+        )
+
     # -- Private -------------------------------------------------------
 
     def _auto_resume(self) -> int: