SDFormat->MJCF: Use unique names for lights

[azeey]

🦟 Bug fix

Summary

Discovered in a local roundtrip test on humanoid.xml

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [x] codecheck passed (See contributing)
• [x] 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.

[codecov[bot]]

Codecov Report

Merging #65 (22099f2) into main (043ad60) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #65   +/-   ##
=======================================
  Coverage   97.22%   97.22%           
=======================================
  Files          19       19           
  Lines         828      828           
=======================================
  Hits          805      805           
  Misses         23       23           

Impacted Files	Coverage Δ
...ormat_to_mjcf/sdformat_to_mjcf/converters/light.py	100.00% <ø> (ø)

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