Affected: getkin kin-openapi

kin-openapi is a Go project for handling OpenAPI files. Prior to 0.146.0, openapi3filter.ValidateResponse passes a nil response header schema to decodeValue when an OpenAPI document declares the header with content instead of schema. The document passes validation, but every response validation for the affected operation dereferences the nil schema and panics even when the response does not contain the declared header. Applications that ingest an untrusted specification can expose this as remote availability impact, while applications using only trusted local specifications still crash when the affected operation is exercised. This issue is fixed in version 0.146.0.

Affected: Go: github.com/getkin/kin-openapi

## Summary

`ValidateResponse` panics on a nil pointer when a spec declares a response header with `content` instead of `schema`. `doc.Validate()` accepts that document, so the crash comes from a spec the library just called valid.

Same class as GHSA-jpcw-4wr7-c3vq, which covered the request-parameter case in v0.144.0; this is the response-header path.

Affects `github.com/getkin/kin-openapi` `<= v0.145.0`. Reproduced on master `947498e5a6bd`, Go 1.26.

## Details

A `content`-typed response header has `headerRef.Value.Schema == nil`.

`validate_response.go:181` passes it straight through:

```go if decodedValue, found, err = decodeValue(dec, headerName, sm, headerRef.Value.Schema, headerRef.Value.Required); err != nil { ```

and `req_resp_decoder.go:273` is where it lands:

```go if len(schema.Value.AllOf) > 0 { ```

Two things I checked rather than assumed. The response does not need to carry the header; declaring it is enough, and then every call for that operation panics. And the body path is fine, `:114` returns early on a nil schema, so this is headers only. The PoC keeps the body case as a control.

## PoC

Drop into `openapi3filter/` and run:

``` go test ./openapi3filter/ -run NilSchema -v ```

```go package openapi3filter_test

import ( "context" "io" "net/http" "net/http/httptest" "strings" "testing"

"github.com/getkin/kin-openapi/openapi3" "github.com/getkin/kin-openapi/openapi3filter" "github.com/getkin/kin-openapi/routers" )

const specBody = ` opena…