fix(slack): thread channel ID through inbound context for reactions (#34831)

Slack reaction/thread context routing fixes via canonical synthesis of #34831.

Co-authored-by: Tak <tak@users.noreply.github.com>

src/auto-reply/reply/agent-runner-utils.ts

@@ -58,6 +58,7 @@ export function buildThreadingToolContext(params: {
         ReplyToId: sessionCtx.ReplyToId,
         ThreadLabel: sessionCtx.ThreadLabel,
         MessageThreadId: sessionCtx.MessageThreadId,
+        NativeChannelId: sessionCtx.NativeChannelId,
       },
       hasRepliedRef,
     }) ?? {};