[Console] Still unable to submit valid request bodies, 400 Bad Request Illegal unquoted character, this time for template variables with defaults

[MichaelRisthausSVA]

Kibana version: 8.16.1

Elasticsearch version: 8.16.1

Server OS version: Running in docker container

Browser version: All browsers affected

Browser OS version: win11

Original install method (e.g. download page, yum, from source, etc.): elk container bundle

Describe the bug: In followup to issue #200201 I still have the exact same problem with default values for query template variables. Something like ´{{var}}{{^var}}default value{{/var}}` leads to exact the same error as described in #200201, although I updated my local environment to 8.16.1. As soon as I put an circumflex ^ or a forward slash / in the query template, the parsing error still occurs:

"error": {
    "root_cause": [
      {
        "type": "x_content_parse_exception",
        "reason": "[36:26] [stored script source] failed to parse field [source]"
      }
    ],
    "type": "x_content_parse_exception",
    "reason": "[36:26] [stored script source] failed to parse field [source]",
    "caused_by": {
      "type": "unchecked_i_o_exception",
      "reason": """com.fasterxml.jackson.core.JsonParseException: Illegal unquoted character ((CTRL-CHAR, code 10)): has to be escaped using backslash to be included in string value
 at [Source: (byte[])"{
[...]

Steps to reproduce:

1. Use elastic/kibaba 8.16.1
2. create a query tempate with default values for parameters

PUT _scripts/flight-search-template
{
  "script": {
    "lang": "mustache",
    "source": {
      "from": 3,
      "size": 10,
      "fields": [
        "OriginCountry",
        "DestCountry",
        "DestWeather",
        "OriginWeather",
        "DistanceMiles",
        "timestamp",
        "Dest"
      ],
      "_source": false,
      "highlight": {
        "fields": {
          "Dest": {
            "pre_tags": "<strong>",
            "post_tags": "</strong>"
          }
        }
      },
      "query": {
        "bool": {
          "must": [
            {
              "match": {
                "Dest": "{{query_item}}"
              }
            },
            {
              "range": {
                "timestamp": {
                  "gte": "{{start-date}}{{^start-date}}2024-01-01T00:00:00{{/start-date}}",
                  "lte": "{{end-date}}{{^end-date}}now{{/end-date}}"
                }
              }
            }
          ],
          "must_not": [
            {
              "match": {
                "OriginWeather": "Rain"
              }
            }
          ],
          "should": [
            {
              "match": {
                "DestWeather": "Sunny"
              }
            }
          ]
        }
      },
      "sort": [
        {
          "timestamp": "desc"
        }
      ]
    }
  }
}

Expected behavior:

Query template should be accepted as it is (and as eg elastic 8.13.3 does)

Screenshots (if relevant): n/a

Errors in browser console (if relevant): n/a

Provide logs and/or server output (if relevant): n/a

Any additional context: n/a

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)

[sabarasaba]

We missed the 8.16.1 by just a few days which is why you are still not seeing the change there, it will be included in 8.16.2. I double checked your example and seems to work: