Fix Json column ReadForeign to parse JSON strings

[i-am-the-slime]

Summary

• Fix ReadForeign Json instance to call parseJSON on the string value returned by SQLite, instead of wrapping the raw string as Foreign
• Update Json round-trip test to verify parsed content matches the original object

Problem

When selecting a Json column, SQLite returns the value as a TEXT string. The ReadForeign Json instance was wrapping this string directly (pure <<< Json), so the Json newtype contained a string rather than a parsed JSON object.

Fix

Follow the same pattern as SQLDate/SQLTime — first read the Foreign as a String, then parse it with parseJSON from yoga-json.

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