DEV: Correct service-worker sourceMappingURL (#15916)

We serve `service-worker.js` in an unusual way, which means that the sourcemap is not available on an adjacent path. This means that the browser fails to fetch the map, and shows an error in the console.

This commit re-writes the source map reference in the static_controller to be an absolute link to the asset (including the appropriate CDN, if enabled), and adds a spec for the behavior.

It's important to do this at runtime, rather than JS precompile time, so that changes to CDN configuration do not require re-compilation to take effect.

spec/requests/static_controller_spec.rb

@@ -404,4 +404,27 @@ describe StaticController do
       end
     end
   end
+
+  describe "#service_worker_asset" do
+    it "works" do
+      get "/service-worker.js"
+      expect(response.status).to eq(200)
+      expect(response.content_type).to start_with("application/javascript")
+      expect(response.body).to include("workbox")
+    end
+
+    it "replaces sourcemap URL" do
+      Rails.application.assets_manifest.stubs(:find_sources).with("service-worker.js").returns([
+        <<~JS
+          someFakeServiceWorkerSource();
+          //# sourceMappingURL=service-worker-abcde.js.map
+        JS
+      ])
+
+      get "/service-worker.js"
+      expect(response.status).to eq(200)
+      expect(response.content_type).to start_with("application/javascript")
+      expect(response.body).to include("sourceMappingURL=/assets/service-worker-abcde.js.map")
+    end
+  end
 end