Ensure exploration_results is a numpy array

[r3kste]

This PR ensures that exploration_results in MultistartInfo is always a numpy array.

Summary

1. Fix a case where exploration_results is converted to a list.
2. Test that it is a numpy array of dtype np.float64.
3. Change type hint of exploration_results from list[float] to NDArray[np.float64].

Possible Changes

I would like to get feedback on whether the following changes should be included.

• Use a less strict check for dtype of exploration_results. For example, a subdtype of np.floating
• Add a check to assert that exploration_results is positive for minimize and negative for maximize.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/optimagic/optimization/optimize_result.py	94.16% <100.00%> (+0.04%)	⬆️
src/optimagic/optimization/process_results.py	94.11% <100.00%> (ø)

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