Prevent NPE by using interface rather than Aero casting

[Sleet01]

Fixes #5705 by avoiding cast to Aero class when computing movement envelope.
While LAMs return true to .isAero() when they are in ASF form, they extend BipedMech rather than from Aero so the cast will fail.
But they implement the IAero interface, which has the same methods.

Testing:

• Ran several missions with LAMs in all three modes, including transforming, and performed various velocity changes and maneuvers.
• Ran all 3 projects' unit tests.

Close #5705

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.05%. Comparing base (8b06aec) to head (c37b4d2).
Report is 32 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5813      +/-   ##
============================================
- Coverage     29.06%   29.05%   -0.01%     
+ Complexity    13913    13909       -4     
============================================
  Files          2507     2507              
  Lines        266339   266358      +19     
  Branches      47611    47617       +6     
============================================
- Hits          77403    77398       -5     
- Misses       185012   185023      +11     
- Partials       3924     3937      +13     

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