PR validation Windows leg takes ~50 minutes #699

PR validation builds are slow, particularly the Windows leg which takes ~50 minutes compared to ~20 minutes for Linux and Mac. Example: build 1400224. The Windows pipeline timeout is already configured at 90 minutes.

Observations:

• All tests run sequentially — no parallelism configured (no xunit.runner.json, single dotnet test per platform)
• Each test downloads a full SDK or runtime — 14 Theory methods × 15 channels/10 branches = 100+ individual download-and-verify cycles
• Same channel matrix tested across multiple runtime types — SDK, dotnet, aspnetcore, windowsdesktop all download independently
• PowerShell startup overhead — each test spawns a new powershell.exe process; adds up over 100+ invocations
• Windows runs additional tests — windowsdesktop + tar support tests only execute on Windows

Areas to consider:

• Test parallelism (in-process or pipeline-level)
• Whether every channel × runtime combination is needed
• PowerShell host choice (pwsh vs powershell.exe)
• Separation of fast vs slow tests
• Pipeline structure across multiple agents