TST: cover Function unit conversions

[ting-hong-shieh]

Summary

Add two behavior tests for the Function branch of convert_units():

• convert an input axis from seconds to minutes and verify the source data, axis labels, interpolation, and extrapolation metadata;
• convert a temperature output from kelvin to degrees Celsius and verify the same returned-Function contract.

This changes tests only. It does not modify unit-conversion behavior.

Refs #709.

Revisions

• Base: 62aa0f9be32eeccafbc1aefd6ac3d90306b257ce
• Head: 2bd510ac7a5b93b10b8b03ca117feb75c6597380

Before and after

The same targeted test and coverage command was run at the base and head revisions.

Measurement	Base	Head
rocketpy/units.py statements	48	48
Covered statements	37	48
Missed statements	11	0
Coverage	77%	100%
tests/unit/test_units.py	19 passed	21 passed

Validation

$ python -m pytest tests/unit/test_units.py -q
21 passed in 0.02s

$ python -m pytest tests/unit/test_units.py -q --cov=rocketpy --cov-report=term-missing
21 passed in 1.32s
rocketpy/units.py  48  0  100%

$ python -m ruff check tests/unit/test_units.py
All checks passed!

$ python -m ruff format --check tests/unit/test_units.py
1 file already formatted

The array assertions use NumPy's assert_allclose defaults: rtol=1e-7 and atol=0.

Environment

• Python 3.12.6
• RocketPy 1.13.0
• NumPy 2.5.2
• SciPy 1.18.0
• pytest 9.1.1
• macOS 26.5.2, arm64

Scope

The coverage comparison is limited to the targeted units test file. I did not use this focused run to claim a full non-slow-suite coverage increase.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.51%. Comparing base (e0ff281) to head (2bd510a).
⚠️ Report is 59 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1157      +/-   ##
===========================================
+ Coverage    82.18%   83.51%   +1.33%     
===========================================
  Files          122      130       +8     
  Lines        16355    17082     +727     
===========================================
+ Hits         13441    14266     +825     
+ Misses        2914     2816      -98     

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