Standardize `NVIDIA_API_BASE` environment variable to `NVAI_BASE_URL`

[ashsong-nv]

Description

This PR standardizes the NVIDIA_API_BASE environment variable to NVAI_BASE_URL per discussion with maintainers. The new name better reflects the usage of this API for LLM-related tasks, and disambiguates it from other NVIDIA APIs we may need to use in the future.

I also updated the NVFoundationLLMService docstring with the new default base url used in the langchain-nvidia-ai-endpoints library.

By Submitting this PR I confirm:

• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.