Fix ROS2 RTX radar helper lifecycle guards

[sylvesterkaczmarek]

Description

Fix two early lifecycle guards in OgnROS2RtxRadarHelper.

First, the helper checked state.initialized before inputs:enabled. Once initialized, disabling the node therefore returned immediately without calling custom_reset(), leaving its writer state active.

Second, the helper tested stage.GetPrimAtPath(render_product_path) is None. USD returns an invalid Usd.Prim for a missing path rather than None, so the intended retry guard did not catch a render product that had not been created yet.

The helper now:

• handles enabled=false first and resets initialized state
• validates the render product prim with IsValid() before camera lookup

Validation

• regression test verifies disabling an initialized helper calls custom_reset()
• regression test verifies a missing render product returns before ViewportManager.get_camera()
• normal initialized and valid-render-product paths are unchanged