#21930: Validate Telegram delivery targets to reject invalid formats

## Summary - **Problem:** Cron jobs with Telegram delivery targets using chatId/topicId format (e.g. `-10012345/6789`) were accepted but silently failed - **Why it matters:** Users got confusing behavior with failed/incorrect deliveries - **What changed:** Added validation that specifically rejects `-number/number` format while allowing t.me URLs, @username, and colon-delimited formats - **What did NOT change:** Other channels unaffected; runtime Telegram parsing unchanged ## Change Type - [x] Bug fix ## Scope - [x] Gateway / orchestration ## User-visible / Behavior Changes Error message now shows: `Invalid Telegram delivery target "-10012345/6789". Use colon (:) as delimiter for topics, not slash. Valid formats: -1001234567890, -1001234567890:123, -1001234567890:topic:123, @username, https://t.me/username` ## Security Impact - All No ## Compatibility - Backward compatible: Yes (only rejects invalid inputs that already didn't work) - No config changes needed ## Risks - None