ci: enable --strict flag in presubmit twine_check

## Overview
The presubmit `twine_check` step in `ci/run_single_test.sh` currently runs `twine check dist/*` without `--strict` to avoid false-positive failures caused by missing `long_description_content_type` metadata in existing `setup.py` files (see #18211).
## Goal
Once `long_description_content_type="text/x-rst"` is populated across package `setup.py` files via #18211, re-enable the `--strict` flag in `ci/run_single_test.sh`:
```bash
twine check --strict dist/*

After #18211 populates long_description_content_type in the package setup.py files, open ci/run_single_test.sh and locate the presubmit twine_check command. Enable --strict for the dist/* check, then verify the presubmit check completes successfully with the stricter validation.

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