fix: standardize API responses for unknown routes

- Add a centralized not-found handler returning a consistent 404 response
- Assert the expected error payload for unknown endpoints

src/server.test.ts

@@ -2,6 +2,7 @@ import assert from "node:assert";
 import { after, describe, it } from "node:test";
 
 import { buildApp } from "./server.js";
+import { setServers } from "node:dns";
 
 describe("API endpoints", () => {
   const app = buildApp();
@@ -119,6 +120,9 @@ describe("API endpoints", () => {
     });
 
     assert.equal(response.statusCode, 404);
+    assert.deepEqual(response.json(), {
+      error: "Not Found",
+    });
   });
 
   it("returns a generic 500 response for unexpected error", async () => {

src/server.ts

@@ -24,6 +24,12 @@ export const buildApp = () => {
     });
   });
 
+  app.setNotFoundHandler((_request, reply) => {
+    return reply.code(404).send({
+      error: "Not Found",
+    });
+  });
+
   app.get("/health", async () => ({
     status: "ok",
   }));