This PR adds save functionality to the _MonteCarloPlots.all() method, allowing users to save Monte Carlo histogram plots to disk instead of only displaying them.

Changes Made

• Added filename=None parameter to _MonteCarloPlots.all() method following the existing pattern used in _FlightPlots
• Integrated with the existing show_or_save_plot() helper function for consistent file handling and validation
• Enhanced filename handling for multiple plots by appending the result key name (e.g., results_apogee.png, results_x_impact.png)
• Updated method docstring to document the new parameter and supported file formats
• Added comprehensive unit test to verify the new functionality

Usage Examples

# Save all Monte Carlo result plots
monte_carlo.plots.all(filename="monte_carlo_results.png")
# Save specific result keys
monte_carlo.plots.all(keys=["apogee", "x_impact"], filename="selected_results.png")
# Backward compatibility - existing code still works
monte_carlo.plots.all() # Still displays plots without saving

Key Features

• Backward compatible: Existing calls without filename parameter continue to work unchanged
• Multiple plot support: When saving multiple result keys, each gets a unique filename with the key name appended
• Consistent API: Uses the same filename parameter pattern as other RocketPy plot methods
• File format support: Supports all matplotlib-compatible formats (png, pdf, svg, eps, etc.)
• Proper validation: Leverages existing show_or_save_plot() helper for file validation and directory creation

Testing

• Added unit test test_monte_carlo_plots_all_with_filename that verifies both display and save modes
• Manual testing confirmed proper file creation with correct naming conventions
• All existing tests pass, ensuring no regressions
• Code achieves 10/10 pylint score

This enhancement addresses a gap in the Monte Carlo plotting functionality and provides consistency with other plotting methods in RocketPy.

Fixes #847.

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