[Test] neural: cover providers_digest rotation carryover

[Test] neural: cover providers_digest rotation carryover

Regression test for the symcache-driven profile rotation fix.

Drives a live rspamd + Redis through: train ANN with providers-only
input (metatokens, disable_symbols_input=true) -> verify NEURAL_SPAM /
NEURAL_HAM fire -> mutate set.symbols/set.digest in the scanner worker
(simulates a symcache shift) -> verify inference still fires after the
next check_anns poll.

Pre-fix the mutation pushes the symbol-list Levenshtein distance well
past the 30% tolerance, the worker rejects the trained profile, and
NEURAL_SPAM stops firing.  Post-fix the providers_digest stays
constant and is recognised as the authoritative schema fingerprint, so
the trained ANN is reloaded.

max_trains=1 because metatokens-only scans produce an identical
vector per message and Redis SADD deduplicates — one spam + one ham
scan are enough to fire training.