Validate environment variables strictly

Enforce strict validation of environment variables using Zod schemas with explicit environment-specific rules. Required configurations should be mandatory in production while allowing flexibility in development/test environments.

Key principles:

1. Mark critical variables as required in production
2. Provide clear error messages for missing configurations
3. Allow optional values only in development/test
4. Implement proper fallback mechanisms for optional configs

Example:

export const env = createEnv({
  server: {
    STRIPE_SECRET_KEY: z.string().refine(
      (val) => process.env.NODE_ENV === 'test' || !!val,
      'Missing STRIPE_SECRET_KEY in production',
    ),
    REDIS_URL: z.string().refine(
      (val) => process.env.NODE_ENV === 'test' || !!val,
      'Missing REDIS_URL in production',
    ),
    // Optional in all environments
    LOG_LEVEL: z.string().optional(),
  },
  client: {
    // Required in all environments
    NEXT_PUBLIC_API_URL: z.string().min(1),
  },
  // Validate on app startup
  runtimeEnv: process.env,
});

This approach prevents silent misconfigurations in production while maintaining development flexibility and providing clear error messages when required variables are missing.