[Orbit] Fix type hints in new_best_metric.py

[LINYV0719]

…sing return types

Description

The filename argument in JSONPersistedValue was annotated as str, but the implementation explicitly checks if self._filename is not None, implying it should be Optional[str]
This PR:
Changes filename: str to filename: Optional[str].
Adds missing return type hints (-> Any, -> None) for read and write methods.
Verified with python -m orbit.actions.new_best_metric_test.

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• Documentation update
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Tests

I ran the existing unit tests to verify that the type hint changes didn't break any functionality.

Test Configuration:

OS: Windows
Python Version: Python 3.10
Command: python -m orbit.actions.new_best_metric_test
Result: All tests passed.

Checklist

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• My code follows the coding guidelines.
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• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.

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