add generation cancellation middleware and refactor chat/message workflows to support HTTP client disconnections

src/middleware/generationCancellation.js

@@ -0,0 +1,13 @@
+/** Cancel generation when its HTTP client disconnects, for JSON and streamed replies. */
+export function generationCancellation(req, res, next) {
+    const controller = new AbortController();
+    const onClose = () => {
+        if (!res.writableEnded) controller.abort();
+        res.removeListener("finish", onFinish);
+    };
+    const onFinish = () => res.removeListener("close", onClose);
+    res.once("close", onClose);
+    res.once("finish", onFinish);
+    req.generationSignal = controller.signal;
+    next();
+}