kin-openapi is a Go project for handling OpenAPI files. From 0.136.0 until 0.143.0, openapi3filter accepts OpenAPI 3.1 array schemas without an items keyword and then dereferences the nil Schema.Items value while decoding application/x-www-form-urlencoded or multipart/form-data request bodies, response headers, and other array parameter surfaces. A remote unauthenticated client can reach an affected operation with a nonempty request or matching response and deterministically trigger a nil-pointer panic during ValidateRequest or ValidateResponse. OpenAPI 3.0 documents are not affected because document validation rejects arrays without items, and JSON request bodies use a different decoder path. This issue is fixed in version 0.143.0.

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

### Summary

A nil-pointer dereference in `openapi3filter` lets any unauthenticated client crash a request handler with a single HTTP request against an OpenAPI **3.1** operation that declares `type: array` **without** `items`. OpenAPI 3.1 (JSON Schema 2020-12) permits omitting `items`, and `doc.Validate()` accepts such schemas with `Schema.Items == nil`. Request/response decoders then dereference `Items` with no nil check. The panic is **deterministic (100%)** on every request to the affected operation. There is no `recover()` in `openapi3filter`; under handlers without per-request recover the process can exit.

### Details

**Spec-validation asymmetry.** OpenAPI 3.0 rejects array schemas that omit `items` ([`openapi3/schema.go`](https://github.com/getkin/kin-openapi/blob/master/openapi3/schema.go) around the `TypeArray` / `newSchemaItemsRequired` branch). For OpenAPI 3.1, `T.Validate` enables `jsonSchema2020ValidationEnabled`, so that check is skipped and `Schema.Items` remains nil after a clean `doc.Validate()`.

**Vulnerable decoder.** The primary request-side crash is in [`UrlencodedBodyDecoder`](https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go) (still present on this checkout at L1421):

```go case propType.Is("array"): items := propSchema.Value.Items.Value // Items == nil → panic ```

This runs during schema inspection **before** form values are parsed. Any non-empty `application/x-www-form-urlencoded` body for the operation re…