Extract _scale_url() for overridable scale URL generation

[jensens]

Summary

• Extracts hardcoded @@images URL building into an overridable _scale_url(uid, extension, base_url=None) method on both ImageScale and ImageScaling
• All URL generation in __init__, srcset_attribute(), and srcset() now delegates to _scale_url()
• Custom image backends (e.g. Thumbor) can override a single method to generate direct URLs instead of @@images redirects

Motivation

Currently, scale URLs are hardcoded inline in multiple places as f"{base_url}/@@images/{uid}.{extension}". Custom scaling backends that want to produce different URLs (e.g. direct CDN/Thumbor URLs) must copy entire methods. This refactor provides a clean override point.

See #199 for the original proposal.

Example usage

class ThumborImageScaling(ImageScaling):
    _scale_view_class = ThumborImageScale

    def _scale_url(self, uid, extension, base_url=None):
        return f"https://thumbor.example.com/{uid}.{extension}"

class ThumborImageScale(ImageScale):
    def _scale_url(self, uid, extension, base_url=None):
        return f"https://thumbor.example.com/{uid}.{extension}"

Test plan

• 6 new tests in TestScaleUrl covering default behavior, custom base URL, subclass override on both classes, and srcset integration
• All 126 existing tests pass with 0 failures

Closes #199

[mister-roboto]

@jensens thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[jensens]

@jenkins-plone-org please run jobs

[jensens]

Additional: I first was thinking about using a ZCA utility, but its some overhead to override 3loc in 2 places and IMO not worth the effort.

[erral]

I think it makes total sense to have a method to create such urls

[davisagli]

Makes sense. I do wonder if the entire info should be passed in to _scale_url, in case there are other factors needed to build the url (maybe there is some service that puts the dimensions in the url, for example)