tighten a bit JSONSchema type

Great project! I was playing with adding just enough support in genaiscript to use the agentic function in my script (https://github.com/microsoft/genaiscript/pull/652)

The only issue is a silly typing error because JSONSchema is too loose (https://github.com/transitive-bullshit/agentic/blob/c64069cf9da40536bf2cfe4147ea291ed6700f14/packages/core/src/types.ts#L22) and does not match my own type definition (yay we all have a copy of those)

I was wondering if you could beef up that definition.