Validate RTX lidar render product before camera lookup

[sylvesterkaczmarek]

Description

Fix prerequisite validation in ROS2RtxLidarHelper so missing stages and not-yet-created render products are handled as retryable conditions.

The helper checked stage.GetPrimAtPath(render_product_path) is None, but USD returns an invalid Usd.Prim for a missing path. That allows execution to continue into ViewportManager.get_camera() with a render product that does not exist.

The helper also dereferenced the stage without guarding the case where no USD stage is available yet.

This change:

• returns cleanly when the USD stage is not available
• validates the render-product prim with IsValid()
• prevents camera lookup until the render product actually exists
• leaves the valid RTX lidar setup and publishing paths unchanged

Validation

• regression test covers a missing USD stage
• regression test covers a missing render-product prim
• both verify ViewportManager.get_camera() is not called prematurely
• modified Python files pass py_compile and git diff --check