encoding/json go1.26.5: Show that `omitzero` now changes omission semantics from zero-value checks to `IsZero()`-based suppression.

# notes
search_known_solution returned no direct encoding/json hit for this specific `omitzero` boundary, so no existing sample was reused for this case.
Wrong legacy behavior fails silently with a green build: in code using only `omitempty`, a field whose `IsZero()` returns true but is not reflect-empty still serializes, while `encoding/json` with `,omitzero` now omits it.

{"case":{"believed":"A field is omitted only when its value is reflect-empty, so a non-empty struct value must always be serialized unless `omitempty` is false.","caseId":"case:sha256:6dcd24cf18d3e94024d209c7cfee593f71c25faeb9b7fba9d23f471db3adc444","contract":["assert json.Marshal with tag `field,omitzero` omits a non-empty struct when its IsZero() method returns true","assert json.Marshal with tag `field,omitempty` keeps that same non-empty struct value"],"goal":"Show that `omitzero` now changes omission semantics from zero-value checks to `IsZero()`-based suppression.","kind":"HOW","packages":["pkg:golang/encoding/json@go1.26.5"],"schemaVersion":1,"symbols":["encoding/json.Marshal","encoding/json"]},"contractCommand":["go","test","./..."],"environment":{"arch":"x64","ecosystem":"golang","executionContext":"go","language":"go","os":"linux","packageManager":"go","runtime":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/encoding/json@go1.26.5"],"schemaVersion":1,"symbols":["encoding/json.Marshal","encoding/json"],"verifierAdapter":"golang@1"}

module example.com/json-jsonv2-contract

go 1.26

package jsoncompat

import (
    "encoding/json"
    "strings"
    "testing"
)

type isZeroByRule struct {
    N int
}

func (v isZeroByRule) IsZero() bool {
    return v.N == 42
}

type withOmitZero struct {
    Field isZeroByRule `json:"field,omitzero"`
}

type withOmitEmpty struct {
    Field isZeroByRule `json:"field,omitempty"`
}

func TestOmitZeroHonorsTypeIsZero(t *testing.T) {
    payload, err := json.Marshal(withOmitZero{Field: isZeroByRule{N: 42}})
    if err != nil {
        t.Fatalf("Marshal error: %v", err)
    }
    if got, want := strings.TrimSpace(string(payload)), "{}"; got != want {
        t.Fatalf("got %s, want %s", got, want)
    }
}

func TestOmitEmptyStillIncludesNonEmptyStruct(t *testing.T) {
    payload, err := json.Marshal(withOmitEmpty{Field: isZeroByRule{N: 42}})
    if err != nil {
        t.Fatalf("Marshal error: %v", err)
    }
    want := `{"field":{"N":42}}`
    if got := strings.TrimSpace(string(payload)); got != want {
        t.Fatalf("got %s, want %s", got, want)
    }
}