Add PR gate tests for azd ai agent extension

[v1212]

Summary

Add automated PR gate tests for the \�zd ai agent\ extension using the existing recording/playback proxy infrastructure.

Details

• Tier 0: Offline tests (version, help, flag validation, sample list) — no Azure credentials needed
• Tier 1: Recording playback tests (full \init --no-prompt --project-id\ flow with ARM calls replayed from cassette)
• Negative control test proving cassette consumption (prevents false-green)
• CI workflow integrated into \lint-ext-azure-ai-agents.yml\

Implementation

PR: #8754