[Fix] neural: digest stability under disable_symbols_input

[Fix] neural: digest stability under disable_symbols_input

The profile digest forms part of the Redis key holding the trained
ANN (rn_<rule>_<settings>_<digest>_<v>). process_settings_elt computed
it as lua_util.table_digest(selt.symbols) unconditionally.

With disable_symbols_input=true the symbol catalogue does not feed the
model -- only providers + fusion + max_inputs determine the input-vector
schema (see is_profile_compatible) -- so hashing the unrelated symbol
list rotated the digest whenever any rspamd symbol was added/removed
elsewhere (a new RBL, a multimap rule, an SA-style rule loaded via
multimap's regexp_rules). The trained ANN was orphaned in Redis under
the old key and inference silently dropped to zero hits until a new
sample set retrained from scratch (weeks under realistic class
imbalance). Manual recovery via `redis-cli COPY` of the old key to the
new digest was the only fix.

Now: when has_providers + disable_symbols_input, the digest is
providers_config_digest(rule.providers, rule). Other modes keep the
existing symbol-based digest.

Migration: any deployment already running disable_symbols_input=true
with a trained ANN will see its digest rotate once on first start
after this lands. Either let the model retrain, or use the same
`redis-cli COPY rn_<rule>_<settings>_<old>_<v> rn_<rule>_<settings>_<new>_<v>`
recipe one final time -- after this fix the digest is stable across
unrelated rspamd config changes.