reduce overhead of NPM Last Update badge; test [npm]

[chris48s]

In this PR, I am following up on my suggestion in https://github.com/badges/shields/pull/10641#issuecomment-2453554943

• If the badge URL is /npm/last-update/{packageName} request the abbreviated API response
• If the badge URL is /npm/last-update/{packageName}/{tag} request the full version

This will reduce the amount of json we have to download and parse in order to serve a badge in the case where when a tag is not specified. For packages (e.g: npm) that produce a really large response, it would make it possible to use the tag-less variant.

[github-actions[bot]]

	Warnings
:warning:	:books: Remember to ensure any changes to `config.private` in `services/npm/npm-base.js` are reflected in the [server secrets documentation](https://github.com/badges/shields/blob/master/doc/server-secrets.md)

	Messages
:book:	:sparkles: Thanks for your contribution to Shields, @chris48s!

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