Fix erroneous lookup in github webhook handler.

[blkt]

Summary

While adding tests I erroneously changed a lookup in a package event json from ".package.package_version.container_metadata.tag.name" to ".package.package_version.tag_name", which turned out to be both (a) the wrong thing to do and (b) causing an internal server error because "tag_name" field is not available (anymore?) for package events.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 51.921%. remained the same
when pulling 5419a37 on fix-webhook-lookup-failure
into e707a98 on main.