[New Feature]: Zero-downtime deployment for validation pipeline

### Description of the new feature/enhancement

## Problem

Current deployment infrastructure requires pausing all pipelines while new code is deployed. This causes a **~2-6 hour service disruption** as we wait for validations to complete to continue the deployment process.

After the deployment, the pause lifts and a burst of queued work hits the freshly deployed service simultaneously, causing transient failures and delays.

## Goal

Enable code deployments without interrupting the validation pipeline. PRs should continue to be validated while new code is being deployed.

## Scope

Targets validation pipeline only. Depends on GitHub App Validation being fully operational. #263724

### Proposed technical implementation details (optional)

_No response_