feat(agents): flush reply pipeline before compaction wait (#35489)

Merged via squash.

Prepared head SHA: 7dbbcc510b
Co-authored-by: Sid-Qin <201593046+Sid-Qin@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

src/agents/pi-embedded-runner/run/attempt.ts

@@ -1688,6 +1688,14 @@ export async function runEmbeddedAttempt(
         const preCompactionSessionId = activeSession.sessionId;
 
         try {
+          // Flush buffered block replies before waiting for compaction so the
+          // user receives the assistant response immediately.  Without this,
+          // coalesced/buffered blocks stay in the pipeline until compaction
+          // finishes — which can take minutes on large contexts (#35074).
+          if (params.onBlockReplyFlush) {
+            await params.onBlockReplyFlush();
+          }
+
           await abortable(waitForCompactionRetry());
         } catch (err) {
           if (isRunnerAbortError(err)) {