TELEGRAM_ALLOWED_USERS was only checked for callback/inline-button
actions but not for inbound messages. Unauthorized users triggered an
'Unauthorized user' log warning but their messages were still processed
by the agent — a P0 security bypass (issue #23778).
Fix: add allowlist check in _should_process_message() which is called
for all message types (text, command, media, location). If the sender
is not in TELEGRAM_ALLOWED_USERS, the message is dropped immediately
with a warning log. Empty TELEGRAM_ALLOWED_USERS continues to allow
all users (existing behavior).
Fixes#23778
db50af910b