Fix omitzero handling

[hi-rai]

What changed

• isOptional now marks struct fields tagged omitzero as optional. This includes time.Time and inline structs. Struct fields tagged only omitempty stay required.
• A new helper, hasJSONOption, reads the options of a json tag. Before this change, the code used strings.Contains on the whole tag. isOptional and isNullable both use the new helper.
• getValidateKeys now uses strings.Cut instead of SplitN. This is a small cleanup with no change in behavior.
• Tests and golden files now cover omitzero on structs, time.Time, *time.Time, arrays, slices, maps, pointer-to-slice/map, and fields named omitzero/omitempty.

Why

• encoding/json omits a struct field tagged omitzero when the value is zero or when its IsZero() method returns true. The old code never marked struct fields as optional, so the schema required fields that Go may leave out of the JSON.
• The old substring check also treated a field name as an option. For example, json:"omitzero" sets the field name to omitzero but has no options, and it was still treated as optional.

Impact

• The generated schemas now match what encoding/json produces for these fields. Struct and time.Time fields tagged omitzero now get .optional().
• A field whose JSON name contains omitempty or omitzero is no longer marked optional or non-nullable by mistake.
• The generated output changes for affected types. Consumers should regenerate their schemas. Fields tagged omitzero with validate:"required" stay required.