Preview tokens 6a: Fix preview token of the site model

[lukasbestle]

🚨 Merge first

• Preview tokens 6: Version-based page caching #6827

Description

Summary of changes

In PR 3 I introduced the new $version->previewToken() method. While writing tests for PR 7 I realized that giving the site model its own preview token was wrong. The site preview renders the home page by default, so it also needs to use the home page's preview token by default.

Additional context

This is an "intermediary" PR that gets its name 6a so that we can continue with PR 7 with its original intended contents I already announced.

Changelog

None (internal change before any release)

Docs

None

Ready?

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

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion

[bastianallgeier]

Question: What happens when the site or a page sets a completely different preview URL in its blueprint? Does this matter here at all?

[lukasbestle]

Very good question. But also simple to answer:

Those custom blueprint-defined previews will follow in the last PR 9 and will be implemented by constructing the token from the raw URI. Model-based tokens will still be relevant for the default case as they are more robust and reliable if no URI-based token is needed.