feat(telegram): add linkPreview config option

Add channels.telegram.linkPreview config to control whether link previews
are shown in outbound messages. When set to false, uses Telegram's
link_preview_options.is_disabled to suppress URL previews.

- Add linkPreview to TelegramAccountConfig type
- Add Zod schema validation for linkPreview
- Pass link_preview_options to sendMessage in send.ts and bot/delivery.ts
- Propagate linkPreview config through deliverReplies callers
- Add tests for link preview behavior

Fixes #1675

Co-Authored-By: Claude <noreply@anthropic.com>

src/config/zod-schema.providers-core.ts

@@ -125,6 +125,7 @@ export const TelegramAccountSchemaBase = z
     reactionNotifications: z.enum(["off", "own", "all"]).optional(),
     reactionLevel: z.enum(["off", "ack", "minimal", "extensive"]).optional(),
     heartbeat: ChannelHeartbeatVisibilitySchema,
+    linkPreview: z.boolean().optional(),
   })
   .strict();