Examples validation does not conform to OpenAPI 3.0 standard in case of required property marked as readOnly

[bravenut]

Hey,

we noticed a bug in the examples validation today and created the following minimal reproducible example specification:

openapi: 3.0.1
info:
    description: Test API
    title: Test API
    version: "1"
servers:
    - description: Some backend server
      url: https://some.backend.server
paths:
    /v1/test:
        post:
            description: Test endpoint
            requestBody:
                content:
                    application/json:
                        schema:
                            type: object
                            properties:
                                ID:
                                    $ref: '#/components/schemas/ID'
                            required:
                              - ID
                        example:
                            ID:
                                someOtherId: test 
                required: true
            responses:
                "200":
                  description: success
components:
    schemas:
        ID:
            description: Some ID
            properties:
                someId:
                    type: string
                    readOnly: true
                someOtherId:
                  type: string
            required:
                - someId
                - someOtherId
            type: object

According to the OpenAPI 3.0.0 specification, the readOnly field within the someId property of the ID schema should lead to the example not requiring the someId property, i.e.

readOnly: Declares the property as "read only". This means that it MAY be sent as part of a response but SHOULD NOT be sent as part of the request

Still, the validation fails with

invalid paths: invalid path /v1/test: invalid operation POST: invalid example: Error at "/someID/someId": property "someId" is missing

Although, according to the OpenAPI specification standard, the property should not be part of the request. Instead, adding the someId property to the example erroneously fixes the validation.

[danicc097]

Your example is validated correctly by:

package openapi3_test

import (
	"testing"

	"github.com/stretchr/testify/require"

	"github.com/getkin/kin-openapi/openapi3"
)

func TestIssue1012(t *testing.T) {
	spec := `
openapi: 3.0.1
info:
    description: Test API
    title: Test API
    version: "1"
servers:
    - description: Some backend server
      url: https://some.backend.server
paths:
    /v1/test:
        post:
            description: Test endpoint
            requestBody:
                content:
                    application/json:
                        schema:
                            type: object
                            properties:
                                ID:
                                    $ref: '#/components/schemas/ID'
                            required:
                              - ID
                        example:
                            ID:
                                someOtherId: test 
                required: true
            responses:
                "200":
                  description: success
components:
    schemas:
        ID:
            description: Some ID
            properties:
                someId:
                    type: string
                    readOnly: true
                someOtherId:
                  type: string
            required:
                - someId
                - someOtherId
            type: object
`[1:]

	sl := openapi3.NewLoader()
	doc, err := sl.LoadFromData([]byte(spec))
	require.NoError(t, err)
	require.NotNil(t, doc.Paths)

	err = doc.Validate(sl.Context, openapi3.EnableExamplesValidation())
	require.NoError(t, err)
	require.NotNil(t, doc.Paths)
}

There must have been some regression because it does fail with err = doc.Validate(sl.Context). When no option is passed, the ctx is empty so it is not validating as neither response not request somehow

Update: this issue starts happening with #692