Added Safety Check for ENV Variables

[SaisakthiM]

Add Safety Checks for Critical Environment Variables in fetchAPI

This PR improves the fetchAPI utility by adding explicit validation for critical environment variables. Previously, missing GRAPHCMS_PROJECT_API or auth tokens could lead to cryptic errors during runtime. With this change:

• GRAPHCMS_PROJECT_API is validated at the start of the function.
• GRAPHCMS_DEV_AUTH_TOKEN and GRAPHCMS_PROD_AUTH_TOKEN are validated based on the preview flag.
• Clear and descriptive error messages are thrown if any required environment variable is missing.
• Maintains the original API fetching logic and error handling for json.errors.

Benefits

• Provides early, clear feedback to developers when environment variables are misconfigured.
• Prevents runtime errors caused by undefined or missing environment variables.
• Improves code readability and maintainability.

Example Error Messages After This PR

• Missing GRAPHCMS_PROJECT_API environment variable
• Missing GRAPHCMS_DEV_AUTH_TOKEN environment variable
• Missing GRAPHCMS_PROD_AUTH_TOKEN environment variable

This enhancement increases the robustness of the API utility and follows best practices for handling critical environment dependencies.