Default values missing from OpenAPI output

[lcawl]

Priority: high
effort: low?

Request

The OpenAPI generator should include default values in the output files.

NOTE: If there are circunstances where the default value might differ in the Serverless context, should the @server_default also be changed to be context-aware (e.g. similar to the @availability option)?

Motivation

The specifications contain @server_default values, but they're not being propagated to the OpenAPI output.

In Kibana API documents, those default fields are present in the OpenAPI, for example:

      parameters:
        - description: The number of rules to return per page.
          in: query
          name: per_page
          required: false
          schema:
            default: 10
            minimum: 0
            type: number

... which then is displayed in our docs like this (https://www.elastic.co/docs/api/doc/kibana/operation/operation-get-alerting-rules-find):

Example

For example, looking at https://github.com/elastic/elasticsearch-specification/blob/main/specification/esql/query/QueryRequest.ts, which has @server_default values set like this:

    /**
     * When set to `true` and performing a cross-cluster query, the response will include an extra `_clusters`
     * object with information about the clusters that participated in the search along with info such as shards
     * count.
     * @server_default false
     */
    include_ccs_metadata?: boolean

I can see that same information carried into the specification files, e.g. serverDefault appears for include_ccs_metadata in https://raw.githubusercontent.com/elastic/elasticsearch-specification/refs/heads/main/output/schema/schema-serverless.json

          {
            "description": "When set to `true` and performing a cross-cluster query, the response will include an extra `_clusters`\nobject with information about the clusters that participated in the search along with info such as shards\ncount.",
            "name": "include_ccs_metadata",
            "required": false,
            "serverDefault": false,
            "type": {
              "kind": "instance_of",
              "type": {
                "name": "boolean",
                "namespace": "_builtins"
              }
            }
          }
        ]

However, it's missing from the OpenAPI output. For example:

 "include_ccs_metadata": {
                    "description": "When set to `true` and performing a cross-cluster query, the response will include an extra `_clusters`\nobject with information about the clusters that participated in the search along with info such as shards\ncount.",
                    "type": "boolean"
                  }

It should appear in a default field, as described in https://spec.openapis.org/oas/v3.0.3#properties