Derive default velocity from speed and orientation

[lola831]

Description

Previously, the default/initial velocity was always (0, 0, 0). Now velocity is derived from speed and orientation so it matches the documented behavior.

Issue Link

#339

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

Additional Notes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.91%. Comparing base (f0e3e60) to head (f4d4915).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
- Coverage   89.93%   89.91%   -0.03%     
==========================================
  Files          54       54              
  Lines       13567    13560       -7     
==========================================
- Hits        12202    12192      -10     
- Misses       1365     1368       +3     

Files with missing lines	Coverage Δ
src/scenic/core/object_types.py	93.16% <ø> (ø)

... and 6 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.