add phi-2 model plugin

[mike-ferguson]

Summary

• Adds a new Brain-Score language model plugin for Microsoft's Phi-2 (microsoft/phi-2), a 2.7B parameter causal language model with 32 transformer layers and hidden size 2560.
• Follows the same structure as the existing Gemma plugin, registering the model via HuggingfaceSubject with the last transformer layer (model.layers.31) mapped to the language system.
• Includes tests for model loading, identifier check, neural representation shape, and next-word prediction.

Test plan

• pytest brainscore_language/models/phi2/test.py -m memory_intense passes all four tests
• Model loads successfully with a valid HuggingFace token
• Neural recording returns representations with 2560 neuroid dimensions

[mike-ferguson]

Tests fail due to OOM. Pass locally, so scoring will continue