fix: printSchema in StructuredOutputParser failing when using Zod

[justindra]

When using Zod with the StructuredOutputParser, it always fails as the instanceof checks never passes as instanceof only works when the class used is the exact same one as the one being compared to. This issue occurs when dealing with monorepos where the same package may have multiple instances.

  const parser = StructuredOutputParser.fromZodSchema(
    z.object({
      name: z.string().describe('The name'),
      artists: z
        .array(z.string())
        .describe('An array of artists'),
    })
  );

The error received is:

Error: Cannot read properties of undefined (reading 'typeName')

So using the schema._def.typeName is a lot more reliable in performing these checks.

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[nfcampos]

Thanks @justindra agreed, this is a good fix, I think we should just remove the instanceof checks in favour of this?

[justindra]

@nfcampos yea, I think we can remove the instanceof checks. I'll send through another PR for it.

Update: See #797