Rethink model.key= pattern to better support shared instances

No wait! This won't work, because the model instance is shared across multiple calls - so one user setting the key correctly could leak through to another user.

Originally posted by @simonw in https://github.com/simonw/datasette-llm-embed/issues/3#issuecomment-1751915001

Noticed this while trying to add an API key feature to the datasette-llm-embed plugin.

Since a single embedding model instance is created and then shared by all code that calls llm.get_embedding_model(model_id) - which is good for in-memory models - code that sets model.key = ... is not compatible with shared usage environments such as Datasette plugins where we don't want keys to leak to other users.

So need to reconsider that model.key = ... pattern entirely.

Start by tracing get_embedding_model(model_id) and the shared model.key usage described in the issue. Define a design where API keys supplied by one caller cannot leak through a shared embedding model instance, then verify the behavior across multiple callers and the Datasette plugin use case.