Add dangling instance check for Windows

[scadu]

Summary

This PR extends the dangling instance detection in Elastic CI Mode to support Windows instances. The scaler now correctly detects Windows platform and uses PowerShell to check agent service status.

Changes

• For dangling instance check, replace EC2 instance termination with marking EC2 instance in ASG
• Platform detection:
  • Use strings.EqualFold for case-insensitive comparison (EC2 API returns "windows" lowercase, SDK constant is "Windows")
  • AWS-RunPowerShellScript for Windows, AWS-RunShellScript for Linux
  • Use nssm status buildkite-agent to detect service state
• IAM: add autoscaling:SetInstanceHealth and AWS-RunPowerShellScript to the Lambda role
• Add tests for platform detection, check command generation, and SSM document selection

Testing

To test dangling instance mechanism, I did:

1. Rename terminate-instance.ps1 to terminate-instance.ps1.bak, so it's not triggered via systemd/nssm post-exit event
2. Stopped the buildkite-agent service and let the Agent Scaler do its job

Sample logs:

[Elastic CI Mode] Detected platform: windows
[Elastic CI Mode] Performing detailed SSM check for 1 candidate instance(s): [i-XXXXXXXXXXX]
[Elastic CI Mode] 🧟 Found dangling instance i-XXXXXXXXXXX - buildkite-agent service is not running or check command failed
[Elastic CI Mode] Service status for i-XXXXXXXXXXX: NOT_RUNNING
[Elastic CI Mode] Successfully marked instance i-XXXXXXXXXXX as unhealthy
[Elastic CI Mode] Dangling instance check complete for ASG XXXXXXXXXXX-AgentAutoScaleGroup-XXXXXXXXXXX. Considered: 1, Actually Sent for SSM Check: 1, Marked as Unhealthy this run: 1

[petetomasik]

Some comments/questions added.