Fix construction of nested models

[mjcarroll]

🦟 Bug fix

Summary

This makes dartsim correctly handle nested models. The simulation runner may try to construct models twice (First from the root sdf::Model, and then again because the nested model is in the ECM). This will return the originally constructed model rather than creating a duplicate.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[ahcorde]

linters

[codecov]

Codecov Report

Merging #445 (17fbdeb) into gz-physics6 (96425a2) will increase coverage by 0.00%.
The diff coverage is 83.33%.

❗ Current head 17fbdeb differs from pull request most recent head 6c033ac. Consider uploading reports for the commit 6c033ac to get more accurate results

@@             Coverage Diff              @@
##           gz-physics6     #445   +/-   ##
============================================
  Coverage        75.73%   75.73%           
============================================
  Files              140      140           
  Lines             7034     7039    +5     
============================================
+ Hits              5327     5331    +4     
- Misses            1707     1708    +1     

Impacted Files	Coverage Δ
dartsim/src/SDFFeatures.cc	62.74% <83.33%> (+0.19%)	⬆️

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