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/channels/plugins/types.core.ts

@@ -257,6 +257,8 @@ export type ChannelThreadingContext = {
   ReplyToIdFull?: string;
   ThreadLabel?: string;
   MessageThreadId?: string | number;
+  /** Platform-native channel/conversation id (e.g. Slack DM channel "D…" id). */
+  NativeChannelId?: string;
 };
 
 export type ChannelThreadingToolContext = {