tests: stabilize World::Cloning in asserts-enabled CI

[jslee02]

Stabilize World.Cloning in asserts-enabled (no -DNDEBUG) CI builds.

• Seed dart::math::Random to make the test deterministic.
• Run the long (500-iteration) loop only in DART_BUILD_MODE_RELEASE; use the short loop otherwise (e.g., BUILD_TYPE=None in build-asserts).

---

Before creating a pull request

• Format code using pixi run lint and verify with pixi run check-lint
• Document new methods and classes
• Build with -DDART_TREAT_WARNINGS_AS_ERRORS=ON and resolve warnings

Before merging

• Set milestone
• Update CHANGELOG.md
• Add unit tests
• Add Python bindings (dartpy) if applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.67%. Comparing base (d58fa21) to head (30f6835).
⚠️ Report is 1 commits behind head on release-6.16.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-6.16    #2331   +/-   ##
=============================================
  Coverage         58.67%   58.67%           
=============================================
  Files               384      384           
  Lines             32056    32056           
  Branches           3869     3869           
=============================================
  Hits              18809    18809           
  Misses            13247    13247           

Flag	Coverage Δ
unittests	58.67% <ø> (ø)

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