fix(#3412): swarm-controller answers errors as RFC 9457 problem+json

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.

Cargo.lock

@@ -4591,6 +4591,7 @@ dependencies = [
  "hive-jobq",
  "hive-jobq-wire",
  "hive-types",
+ "problem_details",
  "reqwest 0.13.1",
  "serde",
  "serde_json",