Fix NATS classifier publish dropping messages with quoted multi-word args

Fix NATS classifier publish dropping messages with quoted multi-word args

A shlex-quoted plugin argument (e.g. choose 'multi word option') produces
a routing key containing a literal space. AMQP's topic exchange tolerates
that, but NATS's whitespace-delimited wire protocol does not - the PUB
frame gets corrupted and the server silently drops the connection before
the message reaches any subscriber. Sanitize whitespace out of the
subject before publishing; the JSON payload plugins actually read from is
untouched.

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