Preserve first prim in ROS2 entity traversal

[sylvesterkaczmarek]

Description

Fix get_filtered_entities() so ROS 2 simulation-control entity queries retain the first prim returned by stage traversal.

Stage.Traverse() already iterates real stage prims and does not yield the pseudo-root. The existing code converted traversal to a list and sliced [1:], so it silently discarded the first real prim before applying the /Render exclusion or optional regex filter.

The fix iterates Traverse() directly.

Validation

• regression test verifies the first traversed prim is retained
• /Render prims remain excluded
• regex filtering still applies across all traversed prims
• final diff is one implementation file plus one focused test