🚨 Merge first

• fix: URI builder: Trailing slash on relative URL #7255

Description

Summary of changes

• Use URL instead of relative URI for preview tokens
• Don’t escape slashes in JSON
• Support external preview targets

Reasoning

• We need to use the full URL instead of the relative URI in all cases (even internal previews) to make the behavior consistent and avoid generating different preview tokens for the same target page (one with and one without full URL) – this would mean that validating the token would need to check for both, which would be cumbersome and complex.
• Using the full URL has the disadvantage that the token validation may fail if the base URL is detected differently on rendering the preview than when generating the preview URL. This could in theory happen in exotic setups of the url option, but is not very likely in practice. If it happens, it can be fixed by fixing the custom url setup. This was the reason why I originally decided to use the relative path (which made external previews impossible), but for the headless use-case we need the external preview support and so this solution seems to be the most solid overall.

Changelog

Fixes since v5 beta

• It is now possible again to use external preview URLs [V5] Preview-URL for headless setup #7240

Breaking changes since v5 beta

• Change the underlying logic for preview tokens yet again (breaking preview URLs created in previous v5 pre-releases).

Docs

None

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion