fix: [AG-113] Gemini transformer plugins, incorrect URL and response error parsers

[tysoekong]

Summary

This fixes two related issues:

1. 'Gemini' when using a 'Google One' or 'Personal' subscription, was not forming the AI Transformer URL correctly, and resulted in all 404's
2. 'from_format' parser error handling was using the wrong return argument, resulting in expected #1 got nil errors to the client

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - already up

Issue reference

AG-113