[Phase 1b] End-to-end OOP tests for stateless prompt-retry

[usepowershell]

Part of #98

Validate out-of-process prompt/retry behavior end-to-end.

Scenarios

• OOP prompt-required response is returned correctly
• Retry with valid snapshot completes command
• Invalid/tampered snapshot fails with clear error
• Expired snapshot fails with clear error
• Concurrent OOP requests are isolated
• Connect-AzAccount device code simulation path

Effort: 1 week