fix(charxiv): lazy-init OpenAI client and make model version configur…

[MaxwellJryao]

Summary

• Lazy-initialize OpenAI client to avoid SSLContext pickling errors in datasets.map() multiprocessing
• Make model version configurable via MODEL_VERSION env var for both descriptive and reasoning grading

In scope

• lmms_eval/tasks/charxiv/utils.py: replace module-level client with _get_client() lazy initializer; remove num_proc=1 from dataset.map()
• lmms_eval/tasks/charxiv/reasoning_utils.py: add model parameter to get_reasoning_result_gpt()

Out of scope

• No changes to grading logic, prompts, or evaluation metrics
• No changes to other tasks or model integrations

Validation

Risk / Compatibility

• Low risk: lazy init is functionally equivalent; default model value preserves existing behavior
• MODEL_VERSION env var now takes effect for reasoning grading (previously ignored)

Type of Change

• Bug fix (non-breaking change)
• New feature
• New benchmark/task
• New model integration
• Breaking change
• Documentation update
• Refactoring (no functional changes)