Tolerate json/v2 invalid UTF-8 munging in YAMLToJSON tests

[arthurbdiniz]

What this PR does / why we need it

TestYAMLToJSON's binary data case decodes an invalid UTF-8 payload via a !!binary tag and asserts that the resulting JSON escapes the Unicode replacement character as \ufffd:

"binary data": {
    yaml: "a: !!binary gIGC",
    json: `{"a":"\ufffd\ufffd\ufffd"}`,
    ...
}

Go 1.27 reimplements encoding/json on top of encoding/json/v2. When marshaling a string containing invalid UTF-8 (which decoding !!binary gIGC produces), the standard library now emits the replacement character U+FFFD as raw UTF-8 bytes instead of the \ufffd escape sequence. As a result the test fails on Go 1.27 while passing on Go 1.26 and earlier.

This PR adds a jsonAllow field to the test case struct that lists accepted alternate JSON representations, and tolerates the raw-UTF-8 form for the binary data case. The canonical \ufffd expectation is preserved for every other case, so the byte-exact assertion is unchanged elsewhere. This is a test-only change; there is no change to library behavior.

---

How to reproduce

Using a Go 1.27 toolchain against the current master:

git clone https://github.com/kubernetes-sigs/yaml.git
cd yaml
go1.27 test -run TestYAMLToJSON ./

Output before this change:

--- FAIL: TestYAMLToJSON (0.00s)
    --- FAIL: TestYAMLToJSON/YAMLToJSON (0.00s)
        --- FAIL: TestYAMLToJSON/YAMLToJSON/binary_data_YAMLToJSON (0.00s)
            yaml_test.go:166: Failed to convert YAML to JSON, yaml: `a: !!binary gIGC`, expected json `{"a":"\ufffd\ufffd\ufffd"}`, got `{"a":"<U+FFFD><U+FFFD><U+FFFD>"}`
    --- FAIL: TestYAMLToJSON/YAMLToJSONStrict (0.00s)
        --- FAIL: TestYAMLToJSON/YAMLToJSONStrict/binary_data_YAMLToJSON (0.00s)
            yaml_test.go:166: ...
FAIL
FAIL	sigs.k8s.io/yaml	0.006s

The same command passes on Go 1.24 through Go 1.26, because their encoding/json still emits the \ufffd escape.

---

Prior work

Kubernetes made the equivalent change in k8s.io/apimachinery when adapting to json/v2, using the same "tolerate alternate representation" approach:

• Tolerate json/v2 in unstructured and YAML tests kubernetes/kubernetes#139163 — "Tolerate json/v2 in unstructured and YAML tests"
• Issue: golang tip br0ken! ci-kubernetes-e2e-kind-golang-tip and ci-kubernetes-unit-golang-tip are failing kubernetes/kubernetes#139159

That PR added an allow []string field to the affected test-case struct and accepted the raw-UTF-8 form for the invalid-UTF-8 cases. This PR mirrors that approach for sigs.k8s.io/yaml.

[liggitt]

/lgtm
/approve

[kubernetes-prow]

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This pull-request has been approved by: arthurbdiniz, liggitt

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