perf: bound proxy drain polling

[cgfixit]

Summary

• cap each proxy task-drain sleep to the remaining DrainTimeoutMinutes budget
• preserve the SCCM exit-code contract and PowerShell 5.1 compatibility
• extend the timeout regression test to cover an oversized poll interval

Validation

• Windows PowerShell 5.1 parser: passed
• Pester 5.8.0: 17 passed, 0 failed
• git diff --check: passed
• PSScriptAnalyzer: not installed locally; CI analyzer job is required

No live Veeam, WinRM, SCCM, or proxy operations were run.

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None