fix: suppress reasoning payloads from generic channel dispatch path

When reasoningLevel is 'on', reasoning content was being sent as a
visible message to WhatsApp and other non-Telegram channels via two
paths:
1. Block reply: emitted via onBlockReply in handleMessageEnd
2. Final payloads: added to replyItems in buildEmbeddedRunPayloads

Telegram has its own dispatch path (bot-message-dispatch.ts) that
splits reasoning into a dedicated lane and handles suppression.
The generic dispatch-from-config.ts path used by WhatsApp, web, etc.
had no such filtering.

Fix:
- Add isReasoning?: boolean flag to ReplyPayload
- Tag reasoning payloads at both emission points
- Filter isReasoning payloads in dispatch-from-config.ts for both
  block reply and final reply paths

Telegram is unaffected: it uses its own deliver callback that detects
reasoning via the 'Reasoning:\n' prefix and routes to a separate lane.

Fixes #24954

src/auto-reply/types.ts

@@ -66,6 +66,9 @@ export type ReplyPayload = {
   /** Send audio as voice message (bubble) instead of audio file. Defaults to false. */
   audioAsVoice?: boolean;
   isError?: boolean;
+  /** Marks this payload as a reasoning/thinking block. Channels that do not
+   *  have a dedicated reasoning lane (e.g. WhatsApp, web) should suppress it. */
+  isReasoning?: boolean;
   /** Channel-specific payload data (per-channel envelope). */
   channelData?: Record<string, unknown>;
 };