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());
 		},
 	}));
 };