fix(arr): name the field, not a reply, in the 422

The backend's Malformed Display describes an answer that came back wrong.
Nothing is sent while validating a settings write, so only the reason
belongs in the message.

crates/arr-api/src/subtitle_settings.rs

@@ -107,8 +107,16 @@ impl SubtitleSettingsInput {
         // quietly stopped working. Validated whether or not this build
         // compiled the backend in — the column exists either way.
         if let Some(base_url) = blank_to_none(self.openai_base_url.as_deref()) {
-            arr_subs::OpenAiEndpoint::new(Some(base_url), None)
-                .map_err(|error| format!("openai_base_url: {error}"))?;
+            arr_subs::OpenAiEndpoint::new(Some(base_url), None).map_err(|error| {
+                // The backend's own Display talks about a reply that came
+                // back wrong; here nothing was ever sent, so only the
+                // reason belongs in the message.
+                let reason = match &error {
+                    arr_subs::translate::Error::Malformed { detail, .. } => detail.clone(),
+                    other => other.to_string(),
+                };
+                format!("openai_base_url: {reason}")
+            })?;
         }
         Ok(())
     }
@@ -592,12 +600,9 @@ mod tests {
             .expect("put settings");
         assert_eq!(response.status(), StatusCode::UNPROCESSABLE_ENTITY);
         let body: serde_json::Value = response.json().await.expect("error json");
-        assert!(
-            body["error"]
-                .as_str()
-                .expect("error string")
-                .starts_with("openai_base_url:"),
-            "got {body}"
+        assert_eq!(
+            body["error"], "openai_base_url: bad base URL: relative URL without a base",
+            "the message names the field and the reason, not a reply that never came"
         );
     }