TST: Add unit tests for evaluate_reduced_mass and remove TODO

[Gui-FernandesBR]

Context

This PR resolves a TODO comment in rocketpy/rocket/rocket.py:615: # TODO: add tests for reduced_mass values.

Modifications Checklist

• Remove the # TODO: add tests for reduced_mass values comment in rocketpy/rocket/rocket.py.
• Add three unit tests in tests/unit/rocket/test_rocket.py to cover evaluate_reduced_mass() behavior:
  • test_evaluate_reduced_mass_without_motor: Verifies that calling evaluate_reduced_mass() when the motor is None returns False.
  • test_evaluate_reduced_mass_empty_motor: Verifies that calling evaluate_reduced_mass() when the motor is EmptyMotor returns a Function that evaluates to 0.
  • test_evaluate_reduced_mass_with_motor: Verifies that calling evaluate_reduced_mass() when a motor is associated returns a Function calculating the correct reduced mass.

Expected Outcome

• The TODO comment is cleared.
• evaluate_reduced_mass() is fully covered by unit tests.
• All unit tests pass successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.75%. Comparing base (7e37cb0) to head (f313309).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1051      +/-   ##
===========================================
- Coverage    81.75%   81.75%   -0.01%     
===========================================
  Files          119      119              
  Lines        15192    15201       +9     
===========================================
+ Hits         12420    12427       +7     
- Misses        2772     2774       +2     

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