In multi-account Telegram configurations, `mergeTelegramAccountConfig()`
performs a shallow merge of channel-level config onto each account. This
causes channel-level `groups` to be inherited by ALL accounts, including
those whose bots are not members of the configured groups.
When a secondary bot attempts to handle group messages for a group it is
not in, the failure disrupts message delivery for all accounts — causing
silent message loss with no errors in logs.
Fix: exclude `groups` from the base spread (like `accounts` already is)
and only apply channel-level groups as fallback in single-account setups
for backward compatibility. Multi-account setups must use account-level
groups config.
Added 5 test cases covering single-account inheritance, multi-account
isolation, account-level priority, and backward compatibility.
Fixes#30673
