Missing `correlationData` on disabled Control API endpoints

[vorlif]

Description

The Mosquitto Control API ($CONTROL) generally ensures that responses contain the correlationData provided by the client request, allowing for reliable request-response matching.

However, if a specific endpoint is unavailable—either because the Control API is disabled via enable_control_api false or because a specific plugin (dynamic-security) is not loaded—the error response is minimal and lacks the necessary metadata.

This means that clients that rely on correlationData (e.g., by using UUIDs for asynchronous processing) can no longer map this error back to the original request. This causes the client-side logic to wait indefinitely or time out, as the “connection” to the request is lost in the error message.

Current Behavior

Whenever an endpoint is not reachable/active, the broker returns:

{
  "error": "endpoint not available"
}

Expected Behavior

The error response should adhere to the standard API contract. If correlationData was present in the request, it must be echoed in the error response, even if the command itself was not executed.

Target Format:

{
  "responses": [
    {
      "command": "the-sent-command",
      "error": "endpoint not available",
      "correlationData": "client-provided-data"
    }
  ]
}

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If the maintainers agree, I would also like to submit a PR.