TST: add branch coverage for fin_flutter_analysis and fix keyword argument bug

[erielC]

Pull request type

• Code changes (bugfix, features)
• Code maintenance (refactoring, formatting, tests)

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally

Current behavior

The fin_flutter_analysis function in rocketpy/utilities.py had several untested code branches:

• The see_prints=True branch was never executed in tests
• The see_graphs=True branch was never executed in tests
• Test coverage for utilities.py was at 68%

Additionally, there was a latent bug where the filename parameter was passed as a positional argument to _flutter_plots(), but the function signature defines it as a keyword-only parameter (after *). This would cause TypeError: _flutter_plots() takes 3 positional arguments but 4 were given when users attempted to save flutter plots to files.

New behavior

Tests Added:

1. test_fin_flutter_analysis_with_prints - Tests the see_prints=True branch, ensuring print statements execute correctly and the function returns the expected tuple
2. test_fin_flutter_analysis_with_graphs - Tests the see_graphs=True branch, ensuring plotting functionality works and returns None (uses @patch to mock matplotlib)
3. test_fin_flutter_analysis_complete_output - Tests both flags enabled simultaneously for complete branch coverage

Bugs Fixed:

• Fixed keyword argument bug in fin_flutter_analysis (line 286): Changed _flutter_plots(flight, flutter_mach, safety_factor, filename) to _flutter_plots(flight, flutter_mach, safety_factor, filename=filename)
• Fixed test_flutter_plots to correctly pass filename as a keyword argument

Coverage Impact:

• Increased utilities.py test coverage from 68% to 69%
• +2 statements covered (previously red lines now green)

Breaking change

• No

Additional information

Context: This PR is part of my application for the RocketPy Library Development team (Option 2: Add new tests to improve coverage).

Testing Methodology:

• All new tests use the existing flight_calisto_custom_wind fixture
• Tests use @patch("matplotlib.pyplot.show") to mock matplotlib, preventing plot windows during test execution
• Tests follow existing RocketPy conventions: function-based tests, descriptive docstrings, proper assertions

Quality Checks:

• Ruff: All checks passed
• Pylint: 10.00/10 rating
• Black formatting: Passed
• All unit tests pass locally

Before Tests:

After Tests:

[Gui-FernandesBR]

@erielC congratulations on your first contribution! Code looks clean and good, I appreciate your effort on this, thank you!!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.01%. Comparing base (9cf3dd4) to head (6dd747d).
⚠️ Report is 21 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #904      +/-   ##
===========================================
+ Coverage    80.27%   81.01%   +0.74%     
===========================================
  Files          104      107       +3     
  Lines        12769    13642     +873     
===========================================
+ Hits         10250    11052     +802     
- Misses        2519     2590      +71     

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[Gui-FernandesBR]

Great first PR!