feat: add automatic API key detection from DECART_API_KEY env var

[AdirAmsalem]

Adds support for automatic API key detection from the DECART_API_KEY environment variable.

• Users can now call DecartClient() without arguments if the env var is set
• Explicit apiKey option takes precedence over the env var
• Improved error message guides users to either option

Usage:

# Option 1: Explicit API key
client = DecartClient(api_key="your-api-key")

# Option 2: Using DECART_API_KEY environment variable
client = DecartClient();