Add `QUERY` Support For `application/vnd.apichallenges.todo-query+json`

[eviltester]

I want to send a structured JSON query body to QUERY /todos,
so that I can learn body-based safe querying without needing to learn JSONPath first.

Scope

Implement QUERY /todos with:

Content-Type: application/vnd.apichallenges.todo-query+json
Accept: application/json

The request body should be an API Challenges-defined JSON query object.

Example:

{
  "filter": {
    "doneStatus": false,
    "title": {
      "contains": "scan"
    }
  },
  "sort": [
    {
      "field": "id",
      "direction": "desc"
    }
  ],
  "limit": 10,
  "offset": 0
}

Expected Behaviour

• QUERY /todos is safe and idempotent.
• It must not mutate todo data.
• Supported filter fields:
  • id
  • title
  • description
  • doneStatus
• Supported filter operations:
  • exact match
  • contains for text fields
  • greaterThan / lessThan for id
• Supported sorting:
  • id
  • title
  • doneStatus
• Supported pagination:
  • limit
  • offset
• Response format is controlled by Accept.
• Accept-Query should advertise support:

Accept-Query: application/vnd.apichallenges.todo-query+json

Error Behaviour

• Missing Content-Type returns 400.
• Unsupported Content-Type returns 415.
• Malformed JSON returns 400.
• Unknown query fields or unsupported operators return 422.
• Invalid pagination values return 422.
• Unsupported response Accept values still return 406.

Acceptance Criteria

• A query can filter todos by doneStatus.
• A query can filter text using contains.
• A query can sort and paginate results.
• Unknown fields are rejected rather than ignored.
• The +json media type is parsed as JSON but only accepted when the full media type is supported.
• OPTIONS /todos includes QUERY in Allow.
• Accept-Query includes application/vnd.apichallenges.todo-query+json.