Add explicit return types for plugins that define models, etc

[MichaelDoyle]

I think this will help folks that want to contribute to the plugins with better type safety. I've seen a few errors go by (or almost go by) related to this.

[MichaelDoyle]

This seemed a little funky to me. Should we just make this ChromaRetrieverOptionsSchema?

It looks like we're possibly double-wrapping in an Optional? https://github.com/firebase/genkit/blob/main/js/ai/src/retriever.ts#L164-L168

[ssbushi]

When I first made these abstractions in js/ai, I wanted to make as few assumptions as possible. That meant that the options should not be ZodOptional since we do not know what kind of retriever they are building and what options are required vs optional.

IMO, the /ai abstractions should not add a .optional() on the schema and the providers can add them in the custom schema they define (ChromaRetrieverOptionsSchema in this case).

That being said, this code has through many changes and its evident that it no longer holds to that thought. The problem here does not seem to be double wrapping though -- there is no Optional on the ChromaRetrieverOptionsSchema object and it is added by RetrieverAction.

[MichaelDoyle]

This seemed a little funky to me. Should we just make this PineconeIndexerOptionsSchema?

It looks like we're possibly double-wrapping in an Optional? https://github.com/firebase/genkit/blob/main/js/ai/src/embedder.ts#L74-L78

[MichaelDoyle]

@pavelgj friendly ping on this one