fix(server): abort behavior in sse stream (#12211)

fix AI-121
fix AI-118

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- Improved handling of connection closures and request abortion for
streaming and non-streaming chat endpoints, ensuring session data is
saved appropriately even if the connection is interrupted.
- **Refactor**
- Streamlined internal logic for managing request signals and connection
events, resulting in more robust and explicit session management during
streaming interactions.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/utils.ts

@@ -1,5 +1,7 @@
 import { Readable } from 'node:stream';
 
+import type { Request } from 'express';
+
 import { readBufferWithLimit } from '../../base';
 import { MAX_EMBEDDABLE_SIZE } from './types';
 
@@ -9,3 +11,38 @@ export function readStream(
 ): Promise<Buffer> {
   return readBufferWithLimit(readable, maxSize);
 }
+
+type RequestClosedCallback = (isAborted: boolean) => void;
+type SignalReturnType = {
+  signal: AbortSignal;
+  onConnectionClosed: (cb: RequestClosedCallback) => void;
+};
+
+export function getSignal(req: Request): SignalReturnType {
+  const controller = new AbortController();
+
+  let isAborted = true;
+  let callback: ((isAborted: boolean) => void) | undefined = undefined;
+
+  const onSocketEnd = () => {
+    isAborted = false;
+  };
+  const onSocketClose = (hadError: boolean) => {
+    req.socket.off('end', onSocketEnd);
+    req.socket.off('close', onSocketClose);
+    const aborted = hadError || isAborted;
+    if (aborted) {
+      controller.abort();
+    }
+
+    callback?.(aborted);
+  };
+
+  req.socket.on('end', onSocketEnd);
+  req.socket.on('close', onSocketClose);
+
+  return {
+    signal: controller.signal,
+    onConnectionClosed: cb => (callback = cb),
+  };
+}