Its three error paths returned a bare string with a status code, which forces
a caller to treat the whole body as prose. hive-c0re converted some time ago,
so swarm-controller was the last backend on the old shape -- and it is the one
behind the hive status page's 503, where the body is frequently the entire
diagnosis rather than a summary.
Adds the commitment to docs/conventions.md, since it was implied by the code
in one daemon and written down nowhere: an endpoint of ours answering with a
bare string is a bug to file, not something callers work around.
The test asserts the rendered response -- media type plus an addressable
detail -- rather than the problem_details value, because a handler that built
the value correctly and then returned it as a string would satisfy a test
written against the type alone.
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