Tests expecting json.Marshal to fail on map[interface{}]interface{} fail and panic under Go 1.27

[siretart]

Description

When running the test suite under Go 1.27+, TestNormalizeJSON_ReturnsMarshalError in schema_validation/validate_document_test.go fails and panics with a SIGSEGV nil pointer dereference.

Root Cause

In Go 1.27+, encoding/json.Marshal supports marshaling maps with integer keys (such as map[interface{}]interface{}{1: "one"}) by converting the integer keys to strings ({"1":"one"}), rather than returning an unsupported type: map[interface {}]interface {} error as earlier Go versions did.

Because normalizeJSON(payload) now succeeds without error:

• assert.Error(t, err) fails.
• Line 195 unconditionally evaluates err.Error(), which dereferences a nil pointer and panics.

Stack Trace

=== RUN   TestNormalizeJSON_ReturnsMarshalError
    validate_document_test.go:193: Expected nil, but got: map[string]interface {}{"invalid":map[string]interface {}{"1":"one"}, "openapi":"3.1.0"}
    validate_document_test.go:194: An error is expected but got nil.
--- FAIL: TestNormalizeJSON_ReturnsMarshalError (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x93fc5d]

goroutine 21 [running]:
...
github.com/pb33f/libopenapi-validator/schema_validation.TestNormalizeJSON_ReturnsMarshalError(...)
	schema_validation/validate_document_test.go:195 +0x19d

Affected Tests

The following tests in schema_validation/validate_document_test.go construct map[interface{}]interface{}{1: "one"} expecting json.Marshal / normalizeJSON to produce an error:

• TestNormalizeJSON_ReturnsMarshalError (line 184)
• TestValidateDocument_NormalizationErrorDoesNotValidateNil (line 198)
• TestValidateDocument_CorruptSpecJSONBytesFallbackNormalizationError (line 228)