The generic provider's _native_pricing parses Venice's bespoke model_spec
schema but read only pricing.input/output.usd, dropping cache_input.usd
(e.g. deepseek-v4-1-flash: $0.0075/1M cache reads vs $0.375/1M input).
With input_cache_read left at 0, billing's fallback priced cache reads at
the FULL input rate — a ~50x overcharge on cache hits compared to what the
upstream charges.
cache_input.usd is now coerced with the same rules as the token rates;
a malformed/negative cache rate is treated as absent (never carried),
mirroring the OpenRouter rung's drop-don't-carry behaviour.
Adds regression tests: cache rate carried through, and malformed cache
rates (-, Infinity, non-numeric) dropped while the model still resolves.