bug: SimulationRecorder.save() appends duplicate simulation_end events on repeated calls

[BhoomiAgrawal12]

Describe the bug
SimulationRecorder.save() unconditionally appends a simulation_end event every time it is called. When a user calls save_recording() manually and then the atexit auto-save handler fires at program exit, the output file contains two simulation_end entries.

Expected behavior
simulation_end is recorded exactly once, regardless of how many times save() is called.

To Reproduce

import tempfile
from unittest.mock import MagicMock
from mesa_llm.recording.simulation_recorder import SimulationRecorder

model = MagicMock()
model.steps = 5
model.agents = []
model.__class__.__name__ = "TestModel"
model.max_steps = 10

with tempfile.TemporaryDirectory() as tmp:
    recorder = SimulationRecorder(model=model, output_dir=tmp)
    recorder.save("first_save.json")
    recorder.save("second_save.json")

    ends = [e for e in recorder.events if e.event_type == "simulation_end"]
    print(len(ends))  # this prints 2 where 1 is expected

Additional context
currently, each call to save() appends a new simulation_end event.

I will be raising a pr fixing this.