[Fleet] Cannot create integration with multi secret input

[jillguyonnet]

Ref.: https://elastic.slack.com/archives/CNMNXV4RG/p1734936907785719

Description

There doesn't seem to be an existing case of an integration with an input that has both multi: true and secret: true. The above attempt reports the following Fleet error:

Error creating secrets: ResponseError: x_content_parse_exception Root causes: x_content_parse_exception: [1:10] [post_secret_request] value doesn't support values of type: START_ARRAY

This comes from

kibana/x-pack/plugins/fleet/server/services/secrets.ts

Line 85 in 19f24b3

const msg = `Error creating secrets: ${err}`;

and arises from passing an array value to the secret creation API:

POST /_fleet/secret/
{
  "value": ["mysecret"]
}

I did a local try and found I had a working multi input, but no secrets. I probably configured something differently.

Relevant PRs:
#163075
elastic/elasticsearch#97728

Acceptance criteria

• Validate whether it is possible for an integration to have a multi secret field.
• If so, address above bug.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)