[HTTP/OAS] Add Fleet API descriptions

[lcawl]

Relates to #180056, #181992

This PR adds operation summaries for some Fleet APIs in the generated OpenAPI document. The goal is to replace https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/common/openapi/bundled.json, which I believe is still manually maintained.

How to test

Per #181277:

1. Start Elasticsearch (e.g. yarn es snapshot --license trial)
2. Add server.oas.enabled: true to kibana.dev.yml
3. Start Kibana (e.g. yarn start --no-base-path)
4. curl -s -u elastic http://localhost:5601/api/oas\?pathStartsWith\=/api/fleet | jq > test.json

    "/api/fleet/service_tokens": {
      "post": {
        "summary": "Create a service token",
        "requestBody": {

[apmmachine]

🤖 GitHub comments

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

Thanks @lcawl!

[kpollich]

@elasticmachine merge upstream

[kibana-ci]

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• Commit: 9506f7b

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream