docs: Update test docstrings to follow RocketPy NumPy style guidelines

[Copilot]

Pull request type

• Code maintenance (refactoring, formatting, tests)

Checklist

• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally

Current behavior

Test files created in PR RocketPy-Team#745 (3-DOF simulation feature) lack proper docstrings following RocketPy's NumPy-style documentation guidelines.

New behavior

Added NumPy-style docstrings to all test functions and fixtures in the newly created test files:

• tests/unit/motors/test_point_mass_motor.py - Added docstrings to 5 test functions explaining what each test validates
• tests/unit/rocket/test_point_mass_rocket.py - Added docstrings to 3 test functions with descriptive summaries
• tests/unit/simulation/test_flight_3dof.py - Enhanced docstrings for 2 fixtures and 4 test functions with proper Parameters and Returns sections

Example docstring format used:

def test_structural_and_total_mass():
    """Tests structural and total mass properties of PointMassRocket.

    Verifies that the rocket's structural mass and total mass (including motor
    dry mass and propellant) are calculated correctly at time t=0.
    """

Breaking change

• No

Original prompt

RocketPy-Team#745 update the newly created test files in this PR to write all docstrings according to rocketpy guielines

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