fix nested object json parser issue.

[cb-karthikp]

• Fixes JsonUtil to only extract top-level keys from JSON objects, preventing incorrect value extraction from nested objects/arrays (e.g., id, status, amount inside line_items or
  billing_address being returned instead of the root-level value)
• Replaces per-call regex compilation with a single depth-aware character scanner (findTopLevelValueStart), improving both correctness and performance
• Adds NumberFormatException handling in getLong, getInteger, getDouble, and getBigDecimal — these now return null instead of throwing on malformed/overflow values

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[mpetrik-phrase]

I understand that unwanted recommendation is the worst one... but ... Is it worth fine-tuning a custom JSON parser? There are battle-tested solutions already (with all edge cases covered). Jackson might be too big, but what about GSON or https://github.com/bolerio/mjson?

Is the API size an issue at all? Definitely not for our case. Library reliability definitely is.

[cb-karthikp]

That's a perfectly fair point, and we appreciate the feedback. The library was intentionally designed with zero runtime dependencies to keep it lightweight and avoid potential version conflicts in consumers' projects.