Simplify observability workflow to use only Aspire testing framework

- Removed comprehensive infrastructure validation steps (Docker container startup, health checks, API testing)
- Eliminated manual Aspire environment setup and process management
- Streamlined workflow to focus on Aspire testing framework for automated infrastructure management
- Removed redundant steps like container monitoring, infrastructure validation, and cleanup
- Simplified workflow now follows Microsoft Aspire testing documentation pattern
- ObservabilityMetricsSteps.cs already follows correct DistributedApplicationTestingBuilder.CreateAsync pattern
- Workflow is much cleaner: Setup → Build → Run Aspire Tests → Generate Reports

Co-authored-by: devstress <[email protected]>

Author: Copilot