Fix Gemini Response schema support

[LeonRuggiero]

Gemini models can be forced via configuration to output JSON.
They can also via configuration be forced to follow a specific schema.
The latter is broken, as it expects a internal format which is a subset of OpenAPI 3.0.
See docs: https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/control-generated-output#fields
However we are currently passing it a JSON schema, unmodified.

This is especially causing issues with more complicated schemas, I'm somewhat surprised how well this type mismatch works for simple schemas.

Should be made more robust.