Fix bug in GetUrdfService move_group capability

[sjahr]

Description

The capability did not take into account that it is possible to have link elements that close with "/>". This PR fixes that

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

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (d962501) 50.74% compared to head (0d934f7) 41.08%.
Report is 2 commits behind head on main.

❗ Current head 0d934f7 differs from pull request most recent head 4431877. Consider uploading reports for the commit 4431877 to get more accurate results

Files	Patch %	Lines
...default_capabilities/get_group_urdf_capability.cpp	0.00%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2669      +/-   ##
==========================================
- Coverage   50.74%   41.08%   -9.66%     
==========================================
  Files         392      690     +298     
  Lines       32553    56233   +23680     
  Branches        0     7288    +7288     
==========================================
+ Hits        16517    23099    +6582     
- Misses      16036    32971   +16935     
- Partials        0      163     +163     

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[EzraBrooks]

Reading and writing an XML document with substringing and string concatenation seems very fragile. Using something like TinyXML2 would avoid parsing mistakes like this in the future.

[sjahr]

Reading and writing an XML document with substringing and string concatenation seems very fragile. Using something like TinyXML2 would avoid parsing mistakes like this in the future.

I tried tinyxml2 first but I had a really hard time using it (very little documentation and bad error messaging) and it did not succeed in the end. If we allocate more time for this, I could give it another try but for now I think we should fix this.

[henningkayser]

Isn't there a way to implement the same thing just using urdfdom? It seems very odd to me that we have to run manual xml string parsing and editing.

[sjahr]

#2669 (comment)

Looks like it is possible to export the parsed model to a TiXmlDoment type (tinyxml, not tinyxml2!) but not to a string :/ I guess creating a submodel from a given URDF is not a common usecase 🤷

[marioprats]

This PR LGTM since it's fixing a bug in code that is already there.
Maybe merge this in but open an issue to improve it using urdfdom or existing xml parsing libraries, for when we get to it?