Generalize direction and speed actions with Travels; add MetaDrive Bicycle support

[lola831]

Description

This PR adds a new Travels protocol with direction and speed actions (SetTravelDirectionAction / SetTravelSpeedAction), so agents like bicycles don’t need to use “walking” terminology. Pedestrians continue to use Walks/SetWalking* and existing simulator overrides still work.

This PR also adds MetaDrive bicycle support plus a small bicycle movement test.

Note: This work was prompted by PR #438, which added a MetaDrive Bicycle as a Pedestrian subclass (so bikes were controlled via the walking API).

Issue Link

N/A

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

❌ Patch coverage is 74.07407% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.78%. Comparing base (4be87bd) to head (0c13610).

Files with missing lines	Patch %	Lines
src/scenic/domains/driving/actions.py	74.07%	7 Missing ⚠️

❌ Your patch check has failed because the patch coverage (74.07%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #473      +/-   ##
==========================================
- Coverage   89.85%   89.78%   -0.07%     
==========================================
  Files          48       48              
  Lines       13346    13368      +22     
==========================================
+ Hits        11992    12003      +11     
- Misses       1354     1365      +11     

Files with missing lines	Coverage Δ
src/scenic/domains/driving/actions.py	78.46% <74.07%> (-1.17%)	⬇️

... and 4 files with indirect coverage changes

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