Move macOS UI tests to ACES Shared infrastructure

## Description

Move the public macOS UI-test pool from `Azure Pipelines` / `macOS-14` to `AcesShared` / `ACES_VM_SharedPool_Tahoe`.

PR #34790 attempted this migration, but the macOS lane regressed on the Tahoe pool and cannot be moved yet.

## Current blocker

Compared with the green `macOS-14` baseline, the Tahoe run had failures, hangs, or timeouts across all 23 macOS UI-test jobs:

- Baseline: [`maui-pr-uitests` build 1489272](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1489272)
- Tahoe: [`maui-pr-uitests` build 1488520](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1488520)

Examples include animation/visibility assertion failures and jobs running until worker timeout. The same tests complete successfully on `macOS-14`, pointing to an image, runtime, simulator, or other environment difference on the Tahoe pool.

## Done when

- Identify and resolve the Tahoe image/environment differences causing macOS UI-test failures and hangs.
- Confirm the full macOS UI-test lane runs reliably on `ACES_VM_SharedPool_Tahoe`.
- Update `eng/pipelines/ci-uitests.yml` to use the ACES Shared pool.

Related: #34790

Start with the green macOS-14 build 1489272 and the failing Tahoe build 1488520, comparing their image, runtime, simulator, and worker behavior. Review the macOS UI-test jobs and eng/pipelines/ci-uitests.yml to identify the environment differences. Done means the full lane runs reliably on ACES_VM_SharedPool_Tahoe and the pipeline uses that pool.

Written by the indexing model from the issue text.