bug(tests,constraints): pytest failures on clean run, constructor arg collision, rcParams leak, scalar Quantity indexing

[pankaj-bind]

Running pytest tests/ fails due to three independent bugs:

1. tests/test_route_postprocessing.py:132
boat_speed passed as a positional arg to RoutePostprocessing, but the
constructor is (min_fuel_route, boat, db_engine=None) - causes TypeError: multiple values for argument 'db_engine' on all tests in that class.

2. tests/test_direct_power_method.py:186
test_wind_coeff sets plt.rcParams['text.usetex'] = True and never restores
it, breaking downstream tests with a LaTeX-not-found RuntimeError.

3. WeatherRoutingTool/constraints/route_postprocessing.py:508
calculate_timsestamp() indexes speed[idx] unconditionally - crashes when
speed is a scalar astropy.units.Quantity.

I have a fix ready and will open a PR shortly.

[manavgupta26]

I reproduced these failures locally during setup and confirmed the constructor mismatch and scalar Quantity issue. After adjusting tests and passing per-step speed arrays, the suite stabilizes. Happy to help review the upcoming PR.