test(detector-aws): refactor ECS container ID extraction tests

Refactored test suite to align with existing code style:
- Added helper functions to reduce code duplication
- Improved test organization and descriptions
- Extracted common constants
- Maintained all test functionality while improving readability

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: seongpil0948