#14471: fix(discord): respect replyToMode 'off' in thread/forum channels

When `replyToMode` is `"off"`, replies in Discord thread channels (including forum posts) still get a `message_reference` attached, showing unwanted quote headers. The issue is that `createReplyReferencePlanner` checks `existingId` before checking `replyToMode === "off"`. Thread channels always have an `existingId`, so the setting gets bypassed. Moved the `"off"` guard above the `existingId` check so it actually works regardless of thread context. Added unit tests covering all reply modes. Fixes #14450