test: set _incremental_persistence_failed=False on MagicMock agent

PR #72425 added getattr(agent, '_incremental_persistence_failed', False)
checks at the top of execute_tool_calls_{sequential,concurrent,segmented}.
A bare MagicMock auto-creates a truthy value for any attribute access,
so the interrupt-skip test's MagicMock agent short-circuited before
appending cancelled-tool messages — assert len(messages)==3 got 0.

Production is unaffected: run_conversation resets the flag to False
explicitly at turn start (conversation_loop.py:~1028).

tests/tools/test_interrupt.py

@@ -122,6 +122,11 @@ class TestPreToolCheck:
         agent._interrupt_requested = True
         agent.log_prefix = ""
         agent._persist_session = MagicMock()
+        # PR #72425: execute_tool_calls_* read _incremental_persistence_failed
+        # via getattr at loop top. A bare MagicMock auto-creates a truthy value
+        # for any attribute access, which would short-circuit the interrupt
+        # skip path before any cancelled-tool messages are appended.
+        agent._incremental_persistence_failed = False
 
         # Import and call the method
         import types