fix: serialize NaN values as JSON null

[Saim-naushad]

Fixes #164

Changes proposed in this PR include

• Serialize floating-point NaN values as JSON null for standards-compliant output.
• Preserve backward compatibility when deserializing legacy JSON containing bare NaN.
• Add regression coverage for file and in-memory JSON serialization.
• Add a legacy JSON fixture containing NaN.

Validation

• 619 passed, 1 skipped
• Ruff, mypy, and pylint checks passed.

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