Add schema support

[spinagon]

I added supports_schema = True to llm_openrouter.py, and it works with gemini, openai and mistral right away.
Other models just work as they normally do.
It's possible to requests whether each model+provider supports structured output, but it would take a separate API call for each model.

[simonw]

Relevant docs: https://openrouter.ai/docs/features/structured-outputs

OpenRouter supports structured outputs for compatible models, ensuring responses follow a specific JSON Schema format. This feature is particularly useful when you need consistent, well-formatted responses that can be reliably parsed by your application.

[simonw]

I'm going to enable schemas for all models and mention in the README that some may not work and link to the list on https://openrouter.ai/models?order=newest&supported_parameters=structured_outputs

[simonw]

I was hoping the output of llm openrouters models would include notes on if they support schemas but it does not. I'll file an issue with them about that.

[simonw]

The failure case is a little annoying.

llm -m openrouter/openai/gpt-4o --schema 'name,age int' 'invent a dog'

{"name":"Biscuit","age":5}

But for a model that doesn't support schemas:

llm -m openrouter/google/gemma-2-27b-it --schema 'name,age int' 'invent a dog'

The Noodle Dog

Breed: Noodle

[...]

[simonw]

I'll cover that in the documentation.