[Zod 4] RFC: ZodIssue formats

[colinhacks]

Old formats:
https://github.com/colinhacks/zod/blob/v4-rfc-issues/rfcs/v4-issue-formats/old_formats.ts

export type ZodIssue =
  | ZodInvalidTypeIssue
  | ZodInvalidLiteralIssue
  | ZodUnrecognizedKeysIssue
  | ZodInvalidUnionIssue
  | ZodInvalidUnionDiscriminatorIssue
  | ZodInvalidEnumValueIssue
  | ZodInvalidArgumentsIssue
  | ZodInvalidReturnTypeIssue
  | ZodInvalidDateIssue
  | ZodInvalidStringIssue
  | ZodTooSmallIssue
  | ZodTooBigIssue
  | ZodInvalidIntersectionTypesIssue
  | ZodNotMultipleOfIssue
  | ZodNotFiniteIssue
  | ZodCustomIssue;

---

New formats: https://github.com/colinhacks/zod/blob/v4-rfc-issues/rfcs/v4-issue-formats/new_formats.ts

export type ZodIssue =
  | ZodInvalidTypeIssue
  | ZodInvalidStringIssue
  | ZodInvalidNumberIssue
  | ZodInvalidBigIntIssue
  | ZodInvalidDateIssue
  | ZodInvalidArrayIssue
  | ZodInvalidSetIssue
  | ZodInvalidObjectIssue
  | ZodInvalidFileIssue
  | ZodCustomIssue;

---

Conceptually speaking these are the major changes:

0. ZodIssue is a 2-level discriminated union. The first level discriminates on code: invalid_type, invalid_stringm etc. For each code, there are sub-issues that are discriminated with the check field.
1. Merges invalid_union, invalid_literal, invalid_enum_value into invalid_type
2. Splits up too_big and too_small across the specific types for which they are defined: invalid_string, invalid_array, etc.
3. Within each of the type-specific error code (e.g. invalid_string) there is an additional layer of discrimination using the check key. Instead of a top-level ZodNotMultipleOfIssue, this is now a check: "multiple_of" within code: "invalid_string".
4. Drops invalid_arguments, invalid_return_type, invalid_discriminator_value as these issues are specific to types that will be removed in Zod 4.

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