Fix namespaced robot descriptions in RViz

[Daniiiil1]

Description

Fixes #3799.

The Planning Scene and Motion Planning RViz displays now resolve a relative Robot Description against the configured Move Group Namespace before creating the shared RobotModelLoader. This gives each namespaced display a distinct loader key and subscribes to the matching namespaced robot_description and robot_description_semantic topics. Explicit absolute Robot Description names remain unchanged.

The RDF synchronized-string fallback now also sanitizes its temporary ROS node name. Without this, a namespaced description containing a slash would produce an invalid temporary node name before the topic could be read.

Validation

• clang-format 14.0.6 on all changed C++ files
• git diff --check
• static regression assertions for namespace resolution and sanitized topic-loader path
• Full ROS 2 / RViz runtime validation was not available on this macOS workspace; the draft is ready for CI compilation and maintainer feedback.

Checklist

• Required by CI: code formatted with clang-format 14
• Tutorials/documentation extension is not needed for this bug fix
• Migration note is not needed; existing non-namespaced and absolute-name behavior is preserved
• No screenshot: this changes loading behavior, not the UI
• Full integration test is not included because the existing RDF integration-test harness is currently disabled in CMake