Implement Gemini

[tejanium]

Preamble

I vibe-code this as I don't speak Go, so apologies in advance if the code is not up to standard

What

This PR implements Gemini as one of the Knowledge Graph providers

[Switch-modder]

Would the custom option WirePod already has not work for this?

[tejanium]

I must be doing something wrong, but I use the endpoint from this doc https://ai.google.dev/gemini-api/docs/openai, but could not make it work with the Custom provider

The error I got is

Bot 00501b83 Transcribed text: what's my name
Using gemini-2.5-flash
2025/07/19 08:35:02 Error creating chat completion stream: error, status code: 404, message: EOF

And we have an open issue #387 and they need to use ekatiyar/gemini-openai-proxy to make it work