Initialize the namespace property for custom iMarkers in rviz

[eholum]

Description

MoveIt's iMarker doesn't initialize the marker namespace property, so that functionality is broken when you try to add other iMarkers alongside moveit's planning interface. Normally this just breaks the cursor, though in some cases we've seen rviz crash. I was under the impression that pluginlib might still load the default plugin for other markers, but that does not seem to be the case.

Regardless, I don't think there's a downside to providing the node abstraction to the namespace property object, so this PR just uncomments those lines.

Before (cursor gets stuck as a spinning wheel):

Screen.Recording.2025-08-09.at.10.47.21.AM.mov

After:

Screen.Recording.2025-08-09.at.10.49.49.AM.mov

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.22%. Comparing base (7d6e2ab) to head (513fc43).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ing_rviz_plugin/src/interactive_marker_display.cpp	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3547      +/-   ##
==========================================
- Coverage   46.22%   46.22%   -0.00%     
==========================================
  Files         720      720              
  Lines       62713    62717       +4     
  Branches     7594     7595       +1     
==========================================
  Hits        28985    28985              
- Misses      33562    33566       +4     
  Partials      166      166              

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[v4hn]

This code was copied over from rviz in #342 where @JafarAbdi apparently commented that section to avoid using the rviz node abstraction.
The handle here is only used to select the potential values for the property, so using the rviz handle for that should be fine.