Fix validate-pr-metadata: refresh PR data on rerun and parse trailing pipes

[jolelievre]

Summary

Two fixes to the validate-pr-metadata composite action:

1. Stale data on rerun. The action read PR title / body / milestone / author from $GITHUB_EVENT_PATH, which is the cached event payload at the time the workflow was triggered. When a failed job was rerun after fixing the PR (e.g. updating the title, body, or milestone), the validation kept failing because it never saw the new state. The action now fetches the PR live from the GitHub API on every run, falling back to the event payload only if the API call fails (no token, network error).

2. Trailing pipe in markdown rows. Category and Type were extracted with awk -F'|' '{print $NF}'. A valid markdown row like | Category? | BO | ends with a trailing |, so $NF returned the empty string and the check failed. The action now strips an optional trailing | (and surrounding whitespace) before extracting the value.

Notes for reviewers

• The live API fetch requires a GITHUB_TOKEN to be exposed to the action step (env: GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}). When no token is present, behavior falls back to the previous event-payload reading.
• The companion PR on PrestaShop/PrestaShop updates the calling workflow to pass the token and adds an ordering dependency with the milestone-setting job. Link will be added once opened.