SDFormat to MJCF: Use a unique name for ball joints

[azeey]

🦟 Bug fix

Summary

Some joint types already use unique names, but we missed ball joints.

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)
• [ ] 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.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@55370c2). Click here to learn what that means. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #80   +/-   ##
=======================================
  Coverage        ?   95.16%           
=======================================
  Files           ?       24           
  Lines           ?     1097           
  Branches        ?        0           
=======================================
  Hits            ?     1044           
  Misses          ?       53           
  Partials        ?        0           

Impacted Files	Coverage Δ
...sdformat_mjcf/sdformat_to_mjcf/converters/joint.py	97.56% <100.00%> (ø)

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