Parse SDF mimic joints and retarget mimic pendulum example

[jslee02]

• Parse <mimic> from SDF in dart::utils::SdfParser and apply mimic properties to joints, choosing mimic motor vs coupler constraint from SDF metadata.
• Drop bespoke mimic parsing in the GUI example and integration tests; rely on the parser and retarget all followers to the baseline pendulum while keeping the baseline uncoupled.
• Keep the test/demo world identical to the Gazebo repro SDF and document the change in the changelog.

This aims to address gz-physics issue #432 by reproducing the mimic pendulum scenario via SDF parsing alone.

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Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes (not rerun after the latest merge; previously passed on this branch)
• Add unit tests for new functionality (parser-driven mimic scenario covered by integration test)
• Document new methods and classes (changelog updated; parser behavior aligns with SDF spec)
• Add Python bindings (dartpy) if applicable (not needed here)

[codecov]

Codecov Report

❌ Patch coverage is 90.62500% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.13%. Comparing base (9b39285) to head (c334598).

Files with missing lines	Patch %	Lines
dart/utils/sdf/SdfParser.cpp	90.62%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2254      +/-   ##
==========================================
+ Coverage   61.06%   61.13%   +0.06%     
==========================================
  Files         353      353              
  Lines       32144    32208      +64     
  Branches     4182     4192      +10     
==========================================
+ Hits        19628    19689      +61     
- Misses      12516    12519       +3     

Flag	Coverage Δ
unittests	61.13% <90.62%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
dart/utils/sdf/SdfParser.cpp	81.15% <90.62%> (+0.81%)	⬆️

... and 1 file with indirect coverage changes

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