Validate ROS2 CameraInfo render products before setup

[sylvesterkaczmarek]

Description

Make ROS2CameraInfoHelper treat a missing USD stage or missing render-product prim as a retryable prerequisite instead of continuing into camera-info setup.

Usd.Stage.GetPrimAtPath() returns an invalid Usd.Prim when a path does not exist; it does not return None. The helper was checking is None for both left and right render products, so a not-yet-created render product could pass validation and reach read_camera_info().

The non-SRTX path also constructed Usd.EditContext(stage, stage.GetSessionLayer()) without first checking whether a stage was available.

This change:

• returns cleanly when the USD stage is not available yet
• validates the left render product with Usd.Prim.IsValid()
• validates the optional right/stereo render product the same way
• preserves existing mono, stereo, and SRTX setup once prerequisites are valid

Validation

• regression test covers a missing USD stage
• regression test covers a missing left render product
• regression test covers a missing right/stereo render product
• verifies invalid render products are rejected before read_camera_info()
• modified Python files pass py_compile and git diff --check