fix(response): serialize response data as JSON

[nickita-khylkouski]

Summary

Delegate Resend::Response#to_json to the wrapped response data. This makes direct and nested JSON serialization emit the API data instead of serializing the wrapper's instance variables.

Testing

• bundle exec rake: 405 examples, 0 failures
• RuboCop: 43 files inspected, no offenses

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Summary by cubic

Fixes Resend::Response JSON serialization so to_json returns the API data instead of the wrapper's internal state.

• Direct, nested, and JSON.generate calls now all serialize the response data as expected.
• JSON formatting options like pretty_generate are preserved.

^{Written for commit a5e1e33. Summary will update on new commits.}