♻️ Add defensive defaults and unit tests for content negotiation

Both case expressions dispatching on the negotiated format now
fall back to the transit encoding/format when an unexpected value
shows up, instead of silently returning nil.

Add direct unit tests for app.http.content-negotiation/negotiate-format
covering the Accept header, the _fmt=json query parameter precedence,
and the transit fallback when no explicit signal is present.

AI-assisted-by: glm-5.3-flash

backend/src/app/http/middleware.clj

@@ -178,9 +178,11 @@
                 response)))
 
           (format-response [response request]
-            (case (cnegot/negotiate-format request)
-              :transit (format-response-with-transit response request)
-              :json    (format-response-with-json response request)))
+            (let [format (cnegot/negotiate-format request)]
+              (case format
+                :transit (format-response-with-transit response request)
+                :json    (format-response-with-json response request)
+                (format-response-with-transit response request))))
 
           (process-response [response request]
             (cond-> response

backend/src/app/http/sse.clj

@@ -43,7 +43,8 @@
   [format]
   (case format
     :transit #(t/encode-str % {:type :json-verbose})
-    :json    cnegot/json-encode-str))
+    :json    cnegot/json-encode-str
+    #(t/encode-str % {:type :json-verbose})))
 
 ;; ---- PUBLIC API
 

backend/test/backend_tests/http_content_negotiation_test.clj

@@ -0,0 +1,40 @@
+;; This Source Code Form is subject to the terms of the Mozilla Public
+;; License, v. 2.0. If a copy of the MPL was not distributed with this
+;; file, You can obtain one at http://mozilla.org/MPL/2.0/.
+;;
+;; Copyright (c) KALEIDOS SUBSIDIARY SL
+
+(ns backend-tests.http-content-negotiation-test
+  (:require
+   [app.http.content-negotiation :as cnegot]
+   [clojure.test :as t]))
+
+(t/deftest negotiate-format-defaults-to-transit
+  (t/is (= :transit (cnegot/negotiate-format {})))
+  (t/is (= :transit (cnegot/negotiate-format {:headers {"accept" "*/*"}})))
+  (t/is (= :transit (cnegot/negotiate-format {:headers {"accept" "text/event-stream"}}))))
+
+(t/deftest negotiate-format-from-accept-header
+  (t/is (= :transit (cnegot/negotiate-format
+                     {:headers {"accept" "application/transit+json"}})))
+  (t/is (= :transit (cnegot/negotiate-format
+                     {:headers {"accept" "application/transit+json,text/event-stream,*/*"}})))
+  (t/is (= :json (cnegot/negotiate-format
+                  {:headers {"accept" "application/json"}})))
+  (t/is (= :json (cnegot/negotiate-format
+                  {:headers {"accept" "application/json, text/event-stream"}}))))
+
+(t/deftest negotiate-format-from-query-param
+  (t/is (= :json (cnegot/negotiate-format {:query-params {:_fmt "json"}})))
+  (t/is (= :json (cnegot/negotiate-format
+                  {:query-params {:_fmt "json"}
+                   :headers {"accept" "application/transit+json"}}))))
+
+(t/deftest negotiate-format-only-json-param-is-recognized
+  (t/is (= :json (cnegot/negotiate-format
+                  {:query-params {:_fmt "transit"}
+                   :headers {"accept" "application/json"}})))
+  (t/is (= :transit (cnegot/negotiate-format {:query-params {:_fmt "transit"}}))))
+
+(t/deftest negotiate-format-without-accept-header
+  (t/is (= :transit (cnegot/negotiate-format {:headers {}}))))