fix: emit final 100% download progress for empty response body

onDownloadProgress never fired a completion event when the response
body was empty (e.g. a 200 with Content-Length: 0), since the final
callback in the transform stream's flush() was gated behind a chunk
having been read. Report the final progress unconditionally, falling
back to an empty chunk when the stream produced none.

Closes #878

source/utils/body.ts

@@ -70,8 +70,10 @@ const withProgress = (stream: ReadableStream<Uint8Array>, totalBytes: number, on
 		flush() {
 			if (previousChunk) {
 				transferredBytes += previousChunk.byteLength;
-				onProgress?.({percent: 1, totalBytes: Math.max(totalBytes, transferredBytes), transferredBytes}, previousChunk);
 			}
+
+			// Always report completion, even for an empty stream, so consumers can rely on a final 100% event.
+			onProgress?.({percent: 1, totalBytes: Math.max(totalBytes, transferredBytes), transferredBytes}, previousChunk ?? new Uint8Array());
 		},
 	}));
 };

test/stream.ts

@@ -555,6 +555,26 @@ test('onDownloadProgress cancels original response body', async t => {
 	t.true(didReportProgress);
 });
 
+test('onDownloadProgress emits final 100% progress for empty response body', async t => {
+	const server = await createHttpTestServer(t);
+	server.get('/', (_request, response) => {
+		response.status(200).end();
+	});
+
+	const progressEvents: Progress[] = [];
+	await ky(server.url, {
+		onDownloadProgress(progress) {
+			progressEvents.push(progress);
+		},
+	}).text();
+
+	t.true(progressEvents.length > 0, 'Should emit at least one progress event');
+	const finalProgress = progressEvents.at(-1);
+	t.truthy(finalProgress);
+	t.is(finalProgress!.percent, 1);
+	t.is(finalProgress!.transferredBytes, finalProgress!.totalBytes);
+});
+
 test('forced retry custom request keeps upload progress', async t => {
 	const server = await createHttpTestServer(t, {bodyParser: false});
 	let requestCount = 0;