Vespa CLI visit with `--make-feed` produces invalid JSON for applications with multiple content clusters

[dainiusjocas]

Describe the bug
Vespa CLI visit with --make-feed produces invalid JSON for applications with multiple content clusters.
When two content clusters are available and visit targets all of them:

vespa visit --make-feed -t local | jq

then jq fails to parse JSON with an error:

jq: parse error: Expected separator between values at line 3, column 1

Because the output is:

[
{"id":"id:namespace:doc1::1","fields":{"my_id":"1"}}
{"id":"id:namespace:doc2::2","fields":{"my_id":"2"}}
]

Note the missing comma between docs.

To Reproduce
Here is the reproduction application.

Expected behavior
Output is a valid JSON.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS: [e.g. iOS]
• Infrastructure: Kubernetes, Azure, AWS, self-hosted
• Versions [e.g. 22]

Vespa version
8.691.19

Additional context