[CI/auto-pr] Fix quote heredoc to prevent shell expansion

[x0rw]

This PR fixes an issue with improper quote escaping in the CI workflow that caused unexpected parsing behavior when passing the GitHub event issue payload to the Python script.
Before:

  cat <<EOF | uv run python scripts/auto-pr-helper.py --save
  ${{ toJson(github.event.issue) }}
  EOF

This version failed when the payload contained special characters or quotes.

After:

cat <<'EOF' | uv run python scripts/auto-pr-helper.py --save
${{ toJson(github.event.issue) }}
EOF

By quoting the here-document delimiter with single quotes (<<'EOF'), we ensure the payload is passed literally to the script, preserving all quote characters safely.

Origin:

This was noticed in PR #159, it was missing the code section.

https://github.com/x0rw/safety-critical-rust-coding-guidelines/actions/runs/16647842314/job/47112691866

fixes #161

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[PLeVasseur]

Thank you for the fix @x0rw!