BUG: validation in stochastic/ that survives python -O

[thc1006]

Addresses the TODO at the top of stochastic_model.py, and the point you raised on #1102:

# TODO: Stop using assert in production code. Use exceptions instead.

The reason it matters is that the optimiser removes them, so the checks stop running and malformed input reaches the model instead of being refused:

StochasticModel(obj, mass=("not a number", 0.5))

  python      AssertionError: 'mass': First item of tuple must be an int or float
  python -O   accepted, mass = ('not a number', 0.5, <bound method Generator.normal>)

That model then fails somewhere later with something that does not point back at the tuple that caused it.

Twenty one of them, all in stochastic/:

file	count
stochastic_model.py	13
stochastic_environment.py	3
stochastic_flight.py	3
stochastic_aero_surfaces.py	1
stochastic_parachute.py	1, the one #1103 did not reach

Converted through the AST rather than by hand, so the condition and the message are the ones that were already there. ruff then collapsed not (x in y) to not in where it applied.

AssertionError is kept rather than swapped for TypeError or ValueError. The docstrings document it and callers catching it should keep working. Changing the type is a separate decision from making the check run at all, and I would rather not fold the two together.

Two kinds of test. One reads each module and fails on a reintroduced assert, which is the mechanism. The other runs a child interpreter under -O, because in process the assert is still compiled in and an ordinary test passes either way. Reintroducing one assert fails both, which is how I checked they have teeth.

Also moves two imports in test_custom_sampler.py to the top of the file. pylint reports C0415 on them on develop today, from the cherry-pick of 23be0bab, so pylint rocketpy/ tests/ docs/ currently exits 16 rather than 0.

Local: ruff and format clean, pylint exit 0, 2086 passed and 44 skipped.

[codecov]

Codecov Report

❌ Patch coverage is 21.42857% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.51%. Comparing base (e0ff281) to head (a91c508).
⚠️ Report is 40 commits behind head on develop.

Files with missing lines	Patch %	Lines
rocketpy/stochastic/stochastic_model.py	30.76%	18 Missing ⚠️
rocketpy/stochastic/stochastic_environment.py	0.00%	6 Missing ⚠️
rocketpy/stochastic/stochastic_flight.py	16.66%	5 Missing ⚠️
rocketpy/stochastic/stochastic_aero_surfaces.py	0.00%	2 Missing ⚠️
rocketpy/stochastic/stochastic_parachute.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1111      +/-   ##
===========================================
+ Coverage    82.18%   82.51%   +0.33%     
===========================================
  Files          122      128       +6     
  Lines        16355    16617     +262     
===========================================
+ Hits         13441    13712     +271     
+ Misses        2914     2905       -9     

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