[Fleet] agent_download_sources API resets "is_default" value if it is run twice

[mrodm]

Kibana version: 8.13.0 and 8.14.0-SNAPSHOT

Describe the bug: If the POST agent_download_sources query is executed twice setting with the same body (a new default source with is_default key). Second run of the command fails with a 409 error (expected), but updates the is_default key to be false.

Steps to reproduce:

1. Create a new source using agent_download_sources API:
   • Dev Tools query

     POST kbn:/api/fleet/agent_download_sources
     {
     "id": "test_id",
     "name": "test_name",
     "host": "https://test.url",
     "is_default": true
     }

   • Response:

     {
     "item": {
       "id": "test_id",
       "name": "test_name",
       "host": "https://test.url",
       "is_default": true,
       "proxy_id": null
     }
     }

2. Get the available download sources using GET kbn:/api/fleet/agent_download_sources
   • Dev Tools query:

     GET kbn:/api/fleet/agent_download_sources

   • Response:

     {
     "items": [
       {
         "id": "test_id",
         "name": "test_name",
         "host": "https://test.url",
         "is_default": true,
         "proxy_id": null
       },
       {
         "id": "fleet-default-download-source",
         "name": "Elastic Artifacts",
         "is_default": false,
         "host": "https://artifacts.elastic.co/downloads/"
       }
     ],
     "page": 1,
     "perPage": 10000,
     "total": 2
     }

3. Run again the POST command without is_default key, return 409 error (expected):
   • Dev Tools query

     POST kbn:/api/fleet/agent_download_sources
     {
     "id": "test_id",
     "name": "test_name",
     "host": "https://test.url"
     }

   • Response:

     {
     "statusCode": 409,
     "error": "Conflict",
     "message": """[ingest-download-sources:test_id]: version conflict, document already exists (current version [1]): version_conflict_engine_exception
     Root causes:
         version_conflict_engine_exception: [ingest-download-sources:test_id]: version conflict, document already exists (current version [1])"""
     }

4. Get the available download sources using GET kbn:/api/fleet/agent_download_sources. New source keeps being the default one.
   • Dev Tools query:

     GET kbn:/api/fleet/agent_download_sources

   • Response:

     {
     "items": [
       {
         "id": "test_id",
         "name": "test_name",
         "host": "https://test.url",
         "is_default": true,
         "proxy_id": null
       },
       {
         "id": "fleet-default-download-source",
         "name": "Elastic Artifacts",
         "is_default": false,
         "host": "https://artifacts.elastic.co/downloads/"
       }
     ],
     "page": 1,
     "perPage": 10000,
     "total": 2
     }

5. Run again the same POST command as in first step, return 409 error (expected):
   • Dev Tools query

     POST kbn:/api/fleet/agent_download_sources
     {
     "id": "test_id",
     "name": "test_name",
     "host": "https://test.url",
     "is_default": true
     }

   • Response:

     {
     "statusCode": 409,
     "error": "Conflict",
     "message": """[ingest-download-sources:test_id]: version conflict, document already exists (current version [2]): version_conflict_engine_exception
     Root causes:
         version_conflict_engine_exception: [ingest-download-sources:test_id]: version conflict, document already exists (current version [2])"""
     }

6. Check the available sources again (same query as above):
   • Dev Tools query:

     GET kbn:/api/fleet/agent_download_sources

   • Response: Latest source added changed is_default value to false:

     {
     "items": [
       {
         "id": "fleet-default-download-source",
         "name": "Elastic Artifacts",
         "is_default": false,
         "host": "https://artifacts.elastic.co/downloads/"
       },
       {
         "id": "test_id",
         "name": "test_name",
         "host": "https://test.url",
         "is_default": false,
         "proxy_id": null
       }
     ],
     "page": 1,
     "perPage": 10000,
     "total": 2
     }

Expected behavior:

• Fail with 409 error.
• Keep is_default as true in the new test_id source added.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)