Create a Snowflake session using Personal Access Token (PAT) from
environment variables.
Reads all credentials from environment variables:
- SNOWFLAKE_ACCOUNT: Account identifier (required)
- SNOWFLAKE_USER: Username (required)
- SNOWFLAKE_PAT: Personal Access Token (required)
- SNOWFLAKE_WAREHOUSE: Warehouse (optional)
- SNOWFLAKE_DATABASE: Database (optional)
- SNOWFLAKE_SCHEMA: Schema (optional)
