swarm-controller: GET inference answers configured: null without a key

`GET /api/agents/{name}/inference` answered `"configured": {}` both for
an agent with no key in `agent-inference` and for a stored value with no
fields, so a reader could not tell "the swarm sets nothing" from "a
setting with every field unset". It now reads the key as
`Option<AgentInference>`: an absent or deleted key is `null`, a stored
`{}` stays `{}`. The shared `schedules::read` helper is unchanged.

`swarmctl agent inference get` prints "no swarm setting; the agent runs
on its own model and effort" for `null`; `set` merges into an empty
value in that case, as before.

The agent itself still treats no key and `{}` the same.

Refs #5034

swarmctl/src/inference.rs

@@ -30,7 +30,7 @@ struct Settings {
 /// What `GET` answers: the settings, and what the agent reported running on.
 #[derive(Debug, Deserialize)]
 struct View {
-    configured: Settings,
+    configured: Option<Settings>,
     effective: Option<Effective>,
 }
 
@@ -90,7 +90,7 @@ pub(crate) fn get(socket: &Path, agent: &str) -> Result<()> {
     ))?;
     print!(
         "{}{}",
-        render(&view.configured),
+        render_configured(view.configured.as_ref()),
         render_effective(view.effective.as_ref())
     );
     Ok(())
@@ -108,7 +108,7 @@ pub(crate) fn set(socket: &Path, agent: &str, flags: SetFlags) -> Result<()> {
         None::<&()>,
         "inference-get",
     ))?;
-    let wanted = flags.merge(current.configured)?;
+    let wanted = flags.merge(current.configured.unwrap_or_default())?;
     let stored: Settings = rt.block_on(send(
         socket,
         hyper::Method::PUT,
@@ -134,6 +134,14 @@ pub(crate) fn clear(socket: &Path, agent: &str) -> Result<()> {
     Ok(())
 }
 
+/// The swarm's setting, or one line saying it has none.
+fn render_configured(s: Option<&Settings>) -> String {
+    s.map_or_else(
+        || "no swarm setting; the agent runs on its own model and effort\n".to_owned(),
+        render,
+    )
+}
+
 /// One line per field, naming the unset ones.
 fn render(s: &Settings) -> String {
     const UNSET: &str = "(not set; the agent's own)";
@@ -213,13 +221,24 @@ mod tests {
                 "model_source":"swarm","effort_source":"local_file"}}"#,
         )
         .unwrap();
-        assert_eq!(view.configured.model.as_deref(), Some("opus"));
+        assert_eq!(
+            view.configured.as_ref().and_then(|s| s.model.as_deref()),
+            Some("opus")
+        );
         assert_eq!(
             render_effective(view.effective.as_ref()),
             "running model: opus (swarm)\nrunning effort: medium (local_file)\n"
         );
-        let unknown: View = serde_json::from_str(r#"{"configured":{},"effective":null}"#).unwrap();
+        let unknown: View =
+            serde_json::from_str(r#"{"configured":null,"effective":null}"#).unwrap();
+        assert!(render_configured(unknown.configured.as_ref()).starts_with("no swarm setting"));
         assert!(render_effective(unknown.effective.as_ref()).starts_with("running: not reported"));
+        // A stored setting with no fields still lists every field as unset.
+        let empty: View = serde_json::from_str(r#"{"configured":{},"effective":null}"#).unwrap();
+        assert_eq!(
+            render_configured(empty.configured.as_ref()).lines().count(),
+            3
+        );
     }
 
     #[test]