fix(enterprise): persist llm model temperature in create and update APIs

[zfaustk]

Summary

Fixes #247 by persisting temperature in both enterprise LLM model write paths:

• store temperature when creating /api/enterprise/llm-models
• store temperature when updating /api/enterprise/llm-models/{model_id}
• add focused regression coverage for both code paths

Validation

• pytest tests/test_enterprise_llm_model_api.py