Add automated test infrastructure (PR-gate + live E2E) for the azure.ai.agents extension

[v1212]

Summary

The azure.ai.agents azd extension currently has no automated test coverage gating changes. This issue tracks adding a two-tier test setup:

• Tier 0/1 — PR gate (Add PR gate tests for azd ai agent extension #8754): fast unit/contract tests that run on every PR.
• Tier 2 — live E2E (Add live golden-path (Tier 2) pipeline for azd ai agent extension #8758): a manual/scheduled pipeline that runs the real golden path (init → provision → deploy → invoke → down) against the TME Azure subscription via tmux.

Motivation

• Catch regressions in the extension commands and azd integration before release.
• Validate the full end-to-end experience (including provisioning real Foundry resources) on a cadence, outside the PR critical path.

Scope / tasks

• PR-gate Tier 0/1 tests (Add PR gate tests for azd ai agent extension #8754)
• Live golden-path Tier 2 pipeline (Add live golden-path (Tier 2) pipeline for azd ai agent extension #8758)
• Register the ADO pipeline ext-azure-ai-agents-live and validate the first /azp run
• Confirm the azure-sdk-tests service connection targets the TME subscription with the required RBAC

Notes

Live tests run only on demand (/azp run ext-azure-ai-agents-live) and weekly; they are intentionally not part of the PR gate to keep PR latency low and avoid consuming model quota on every push.