Stabilize parallel Pester smoke discovery

[lipkau]

Summary

• stop sharing the parent PowerShell host with parallel Pester thread jobs
• replay each completed job's buffered output exactly once
• guard the runner against reintroducing the unsafe StreamingHost parameter

Cause

Master smoke run 34568913568 exposed a sporadic Pester 5.9.1 discovery failure while concurrent ThreadJobs shared the parent host. The following master run passed on the same tree, confirming a race rather than a Jira API regression.

This follows the workaround documented in pester/Pester#2383 while preserving throttling, per-file timeouts, result aggregation, and useful CI output.

Validation

• Invoke-Build -Task Lint, Build, Test
• 4,625 checks: 4,556 passed, 69 skipped, 0 failed
• synthetic parallel-runner smoke fixture: one pass, output replayed, zero XML errors
• independent review: no blocking findings

Release intent

Test infrastructure only. No package release.