TST: cover Monte Carlo confidence ellipse helpers

[ting-hong-shieh]

Summary

• add deterministic unit coverage for confidence-ellipse axis calculations using a known covariance matrix
• verify generated apogee and impact ellipse patches preserve centers, standard-deviation scaling, colors, and opacity
• keep the slice limited to tests; production behavior is unchanged

Coverage

Using tests/unit/test_tools.py with rocketpy/tools.py as the measured source:

• merge base 6c156490: 50% (157/313 statements)
• this branch: 57% (178/313 statements)

That covers 21 additional statements in the confidence-ellipse helpers.

Validation

• .venv/bin/python -m pytest tests/unit/test_tools.py -q -p no:cacheprovider — 32 passed
• .venv/bin/ruff check tests/unit/test_tools.py
• .venv/bin/ruff format --check tests/unit/test_tools.py

Relates to #709.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.33%. Comparing base (6c15649) to head (139c5af).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1165   +/-   ##
========================================
  Coverage    84.33%   84.33%           
========================================
  Files          130      130           
  Lines        17258    17258           
========================================
  Hits         14554    14554           
  Misses        2704     2704           

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