feat: Add comprehensive Environment API integration tests with test cases

Add Contentstack018_EnvironmentTest.cs with complete CRUD coverage for Environment API.

- 16 tests: sync/async happy path + negative path scenarios
- Fix API calls to use environment names instead of UIDs for fetch/update/delete
- Includes proper cleanup, unique naming, and error handling
- All tests passing with comprehensive positive/negative validation

Author: OMpawar-21