Stabilize python-can filter reconfiguration under active RX polling

[Copilot]

The python-can test suite was flaky because ThreadSafeBus control operations could block behind the background recv() loop, making filter changes and teardown timing-dependent. This change narrows that race in the adapter and adds regression coverage around the affected path.

• Adapter synchronization

  • Pause the PythonCAN RX thread briefly before bus-level control operations that compete with recv().
  • Apply the same coordination for filter reconfiguration and shutdown.
  • Emit a clearer warning if pause acknowledgment is not observed in time.

• Regression coverage

  • Add a targeted test for filter reconfiguration while RX polling is active.
  • Verify the test exercises live polling rather than an idle interface.

• Behavioral impact

  • No wire-format or transport API changes.
  • Scope is limited to internal coordination around python-can bus control calls.

def filter(self, filters: Iterable[Filter]) -> None:
    self._raise_if_closed()
    ...
    self._pause_rx_thread()
    try:
        self._bus.set_filters(can_filters)
    finally:
        self._resume_rx_thread()