Give the external layer a plugin, so the directory exists in a clone

src/plugins/external/jsonContentType.ts

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+import { fastifyPlugin } from 'fastify-plugin';
+import type { FastifyPluginAsync } from 'fastify';
+
+// The infrastructure layer, and the bottom of the dependency arrow:
+//
+//   routes/  tools/   ->   plugins/app/   ->   plugins/external/
+//
+// Nothing in here knows anything about discounts. It is where the plumbing
+// lives — anything that wraps the outside world, or reaches across every
+// response the way this hook does. Your own external concerns go beside it.
+//
+// This one is lifted from the service this test mirrors: it normalises the
+// `content-type` on JSON responses, which Fastify otherwise sends with a
+// charset appended.
+
+const JSON_CONTENT_TYPE = 'application/json';
+
+const jsonContentType: FastifyPluginAsync = async (fastify) => {
+  fastify.addHook('onSend', (_request, reply, payload, done) => {
+    const contentType = reply.getHeader('content-type');
+
+    if (typeof contentType === 'string' && contentType.startsWith(JSON_CONTENT_TYPE)) {
+      reply.header('content-type', JSON_CONTENT_TYPE);
+    }
+
+    done(null, payload);
+  });
+};
+
+export default fastifyPlugin(jsonContentType, { name: 'jsonContentType' });