Fix NATS silently dropping messages whose routing key has an empty token

Fix NATS silently dropping messages whose routing key has an empty token

bot.py joins shlex tokens with '.' and keeps any punctuation the body had,
so a message ending in a full stop ("<url> ... ich will das.") yields a
routing key ending in '.' - an empty trailing token. AMQP's topic exchange
routed that to '#' fine, but nats-server accepts the PUB without any error
while "classifier.>" never matches it, so the message vanished with nothing
logged on either side. Leading dots and a typed "..." (its own token) fail
the same way.

Collapse runs of dots and trim the ends in _sanitize_subject(), with a
fallback token for a key that reduces to nothing. Empty tokens carry no
information, and the binding keys plugins use still match the trimmed form.

Symptom that surfaced it: a posted YouTube URL followed by a sentence went
unanswered - and with URLResolver deliberately skipping youtube URLs, no
fallback title appeared either.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>