[Bug]: SpecFilter skips the ApiResponse#$ref field

[vprudnikov]

Description of the problem/issue

When Spring's OpenApiHateoasLinksCustomizer calls the SpecFilter#removeBrokenReferenceDefinitions method, SpecFilter doesn't take into account the "$ref" field of the ApiResponse class:

addPathItemSchemaRef: 
...
                for (String keyResponses : op.getResponses().keySet()) {
                    ApiResponse response = op.getResponses().get(keyResponses);
                    if (response.getHeaders() != null) {
                        for (String keyHeaders : response.getHeaders().keySet()) {
                            Header header = response.getHeaders().get(keyHeaders);
                            addSchemaRef(header.getSchema(), referencedDefinitions);
                            addContentSchemaRef(header.getContent(), referencedDefinitions);
                        }
                    }
                    addContentSchemaRef(response.getContent(), referencedDefinitions);
                }
...

    private void addContentSchemaRef(Content content, Set<String> referencedDefinitions) {
        if (content != null) {
            for (MediaType mediaType : content.values()) {
                addSchemaRef(mediaType.getSchema(), referencedDefinitions);
            }
        }
    }

Affected Version

2.2.22

Earliest version the bug appears in (if known):
2.2.22

Steps to Reproduce

Define a controller with a reference to a custom schema like so:

    @Operation(summary = "Finds one sample", responses = {
            @ApiResponse(responseCode = "404", ref = "MyCustomSchema"),
    })

Add your custom schema and response using OpenApiCustomizer like so:

    protected static final String REF_MY_CUSTOM_SCHEMA = constructRef("MyCustomSchema");

    @Bean
    OpenApiCustomizer openApiCustomizer() {
        return openApi -> {
        var components = ofNullable(openApi.getComponents()).orElseGet(Components::new);
        commonResponses().forEach(components::addResponses);
        commonSchemas().forEach(components::addSchemas);

        openApi.setComponents(components);
        };
    }

    protected Map<String, Schema> commonSchemas() {
        return ModelConverters.getInstance().read(MyCustomSchema.class);
    }

    protected Map<String, ApiResponse> commonResponses() {
        return Map.of(
                "MyCustomSchema", new ApiResponse()
                        .description("A custom response")
                        .content(new Content().addMediaType(APPLICATION_JSON_VALUE, mediaTypeOf(REF_MY_CUSTOM_SCHEMA)))
        );
    }

    private static MediaType mediaTypeOf(String simpleRef) {
        return new MediaType().schema(new Schema<>().$ref(simpleRef));
    }

Expected Behavior

The schema is presented in the resulting JSON

Actual Behavior

The schema gets filtered by the SpecFilter