[BUG] ExperimentResults to_json uses locale encoding on Windows

Description

ExperimentResults.to_json and compare_with_baseline open result files with the locale encoding. On Windows that is often cp1252, so UTF-8 identifiers and metadata raise UnicodeEncodeError. json.dump also has no default encoder, so datetime values in metadata raise TypeError.

Steps to Reproduce

from pathlib import Path
from crewai.experimental.evaluation.experiment.result import ExperimentResult, ExperimentResults

results = ExperimentResults(
    [ExperimentResult(identifier="café", inputs={}, score=1, expected_score=1, passed=True)],
    metadata={"note": "résumé"},
)
results.to_json(str(Path("out.json")))

Run with a cp1252 locale.

Expected behavior

Files are written and read as UTF-8. Non-JSON metadata values are stringified instead of crashing.

Screenshots/Code snippets

UnicodeEncodeError: 'charmap' codec can't encode character

Operating System

Windows 11

Python Version

3.12

crewAI Version

main