- Replace OPENAI_APIKEY with OPENAI_API_KEY across all samples
- Replace AZURE_FOUNDRY_OPENAI_APIKEY with AZURE_FOUNDRY_OPENAI_API_KEY
- Ensures consistency with OpenAI's standard naming convention
- Applies to .NET and Python samples
Fixes#1001
Co-authored-by: Alexander Zarei <alzarei@users.noreply.github.com>