Manual backfill workflows for release assets

[shrec]

What

• Added workflow_dispatch tag backfill support to release.yml and packaging.yml
• Release and packaging jobs now checkout RELEASE_TAG explicitly, so current dev workflows can rebuild assets for an existing tag
• workflow_dispatch release runs no longer delete an existing GitHub Release before upload, which makes missing-asset backfills safe
• This unblocks re-running missing v3.50.0 artifacts without cutting a new tag

How to get it

• Branch: dev
• Commit: 5ac8d234
• Intended use: dispatch release.yml and packaging.yml with tag=v3.50.0