[Bug]: @encode on a nullable property emits a schema that rejects null in OpenAPI 3.1 and 3.2 #12015

Describe the bug

With openapi-versions: ["3.1.0"], @encode on a nullable property writes the encoded type and format next to the anyOf, not inside it. That top level type always rejects null. When the encoding changes the type, like unixTimestamp, seconds or @encode(string) on a number, it can't match the anyOf either, so every value fails. When the type stays a string, like rfc7231 on utcDateTime, only null fails. 3.2.0 does the same. 3.0.0 is fine, and so is the property without | null. Happy to send a PR.

Reproduction

model Order {
  @encode(DateTimeKnownEncoding.unixTimestamp, int64) shippedAt: utcDateTime | null;
}

3.1.0 output. Ajv (draft 2020) rejects both 1726000000 and null:

{ "anyOf": [{ "type": "string", "format": "date-time" }, { "type": "null" }], "type": "integer", "format": "int64" }

Expected, matching 3.0.0:

{ "anyOf": [{ "type": "integer", "format": "unixtime" }, { "type": "null" }] }

Nullable query parameters fail the same way. bytes | null with base64url only rejects null, but it gets format where plain bytes gets contentEncoding.