Add release-promotion validation for MSBuild telemetry

Related to #14554

### Context

#14554 makes telemetry initialization and disposal best-effort so telemetry failures cannot fail builds. This can silently disable telemetry for an entire release candidate, while unit tests only validate local behavior and cannot verify end-to-end ingestion.

### Proposed Work

- Add a required stage to the release-promotion pipeline.
- Run the exact MSBuild release candidate in a controlled, telemetry-enabled environment.
- Emit a uniquely identifiable canary event without relying on normal event sampling or organic prerelease usage.
- Poll the telemetry backend with an appropriate ingestion timeout and retries.
- Block promotion, or require an explicit manual override, when the canary is not observed.

### Acceptance Criteria

- The candidate version is verified end-to-end before promotion.
- The check does not produce false failures because of sampling or expected telemetry opt-out.
- Failure output identifies the tested version, expected event, and query window.
- The process for overriding a failure is documented.