Cost calculation crashes with AttributeError for unmapped models (e.g. local Ollama models)

## Summary

`fastllm`'s cost tracking crashes for model not present in the bundled
`model_prices.json` . Only a small hand-picked subset of Ollama tags
are keys in that file — e.g. `ollama/llama2` is present, but `ministral-3:3b`
(and many other locally-pulled models) isn't.

## Repro

ssage --vendor_name openai_chat --base_url http://localhost:11434/v1
--api_key ollama --model ministral-3:3b "hi"

The completion succeeds, but crashes right after while tracking usage:

File ".../fastllm/openai_chat.py", line 204, in cost
cost = in_txt * m.input_cost_per_token + out_txt * m.output_cost_per_token
AttributeError: input_cost_per_token

## Cause

`cost()` looks up the model name in `model_prices.json` and assumes the
result always has `input_cost_per_token`/`output_cost_per_token`. There's as far as I can see no
fallback for models that aren't a key in that file.

## Suggested fix

Default to `cost = 0` when a model isn't found, rather than raising. Probably
also add a wildcard fallback for ollama models generally.

Start in fastllm/openai_chat.py around line 204 and inspect how cost() uses the model lookup from model_prices.json. Reproduce with the provided Ollama command and verify that an unmapped model such as ministral-3:3b completes without an AttributeError during usage tracking; determine from the existing lookup whether the wildcard fallback is needed.

Written by the indexing model from the issue text.