add mistral7b model

[mike-ferguson]

Summary

• Adds a new Brain-Score language model plugin for Mistral 7B (mistralai/Mistral-7B-v0.1), a 7.3B parameter causal language model with 32 transformer layers and hidden size 4096.
• Registers the model via HuggingfaceSubject with the last transformer layer (model.layers.31) mapped to the language system.
• Includes tests for model loading, neural representation shape (4096 neuroid dimensions), and next-word prediction.

Test plan

• pytest brainscore_language/models/mistral7b/test.py -m memory_intense passes all three tests
• Model loads successfully with a valid HuggingFace token
• Neural recording returns representations with 4096 neuroid dimensions
• Next-word prediction returns non-empty strings for all inputs