fix: json_serial returns bytes instead of str for non-UTF-8 binary va…

[Shubham-Padkonde]

…lues

When json_serial() receives bytes that cannot be decoded as UTF-8, the fallback path calls base64.b64encode(), which returns bytes, not str. Returning bytes from a json.JSONEncoder default hook causes an immediate TypeError ("Object of type bytes is not JSON serializable") that defeats the entire purpose of the fallback.

Decode the base64 result to an ASCII string before returning so that the value can always be embedded in a JSON document.

Claude-Session: https://claude.ai/code/session_012acaiEt2JyZdUxjQndUWQL

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