* fix: Discord read/search timeout, session-key fallback, and gateway execution mode
- Add 15s timeout to readMessagesDiscord and searchMessagesDiscord so they
fail fast instead of hanging indefinitely (#73431)
- Fall back to CommandTargetSessionKey in dispatchReplyFromConfig when
SessionKey is empty, so Discord inbound message:received hooks fire
reliably (#73431, refs #33038)
- Add resolveExecutionMode to Discord channel actions routing read/search
through gateway timeout path, matching Telegram's pattern (#73431)
* fix: move timeout to fetch layer, drop send.messages wrapper
Inject AbortSignal.timeout into the Discord proxy-request-client fetch
wrapper so every Discord REST call gets a 15s timeout at the HTTP level.
This replaces the Promise.race wrapper in send.messages.ts — cleaner,
covers all calls, and actually aborts the TCP connection.
* fix: remove unused callerController variable in proxy-request-client test
* fix: remove unnecessary mergeAbortSignal helper
