fix(go): Debug logs change from fmt.Sprintf("%#v",d) to serialized JSON in []byte

[peterkarabinovic]

Enhanced Debug Logging

Replaced usage of fmt.Sprintf("%+v", …) for logging complex structs with JSON-marshaled byte arrays.
It allows to analyze and modify log events in the log handler.

Overall Summary

This PR refactors debug logging in Genkit’s core model and action modules to:

• Use JSON serialization for clearer, machine-readable log output.
• Improve consistency and readability of log data for debugging and telemetry.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0
• Tested (manually, unit tested, etc.)

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