fix: loosen the BaseSchema type

[ascorbic]

Changes

Previously BaseSchema used recursive type definitions to try to define "any Zod object, or wrapper type that contains objects". This contained self-referencing types that collapsed to any, meaning there was no typechecking for schema defintions. #13706 fixed that by refactoring out the self-references. However this caused Starlight typechecks to fail (and presumably other sites). At its core, this is because Zod types are inherently recursive, so it's impossible to accurately type them without using recursive types, unless you manually define an arbitrary number of depth of reference. There will always be another layer of abstraction that some user (or more likely integration or library) wants to create.

This PR throws it all away, and simply types BaseSchema as ZodType. This is wider than we actually want: it accepts primitives other than objects. However, so does any, so this is an improvement over that.

Fixes #13713

Testing

The Starlight typecheck that was failing in ecosystem-ci is now passing.

Docs

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[florian-lefebvre]

I'm not sure it's a good idea to allow any zod type. Pretty sure that will lead to confusion. An alternative maybe is to allow any type but check the schema shape when loading the config

[ascorbic]

@florian-lefebvre until yesterday the type was any!

[florian-lefebvre]

True I forgot 😆

[ascorbic]

But I agree that it would to have a runtime check. Also non-trivial, as there's a lot of unwrapping to do and edge cases to handle, so maybe later.

[florian-lefebvre]

Better than any for sure!