fix: ZodFlattenedError accepts a schema, matching inferFlattenedErrors - #6340

Fixes #5029.

z.ZodFlattenedError was a direct alias of core.$ZodFlattenedError, which takes the schema's output type as its generic. The v3 z.inferFlattenedErrors it's meant to replace takes a schema. So a straight migration — inferFlattenedErrors<typeof schema> → ZodFlattenedError<typeof schema> — keyed fieldErrors over the schema's internals (_zod, def, type, ...) instead of the actual fields.

This makes the deprecated alias accept both:

// both now resolve to the same type
type A = z.ZodFlattenedError<typeof schema>["fieldErrors"];
type B = z.ZodFlattenedError<z.output<typeof schema>>["fieldErrors"];
type C = z.inferFlattenedErrors<typeof schema>["fieldErrors"];

When passed a $ZodType, it unwraps to output<T>; otherwise it's treated as a plain output type, so existing callers passing z.output<typeof schema> keep their current typing. Added a type test asserting ZodFlattenedError<typeof schema> is equal to both ZodFlattenedError<output<typeof schema>> and inferFlattenedErrors<typeof schema>.