[6.x] Forms 2: Fix incomplete class error when filtering submissions by site

[jasonvarga]

The forms listing (and the submission site filter) query submissions with where('site', ...). Submissions used the generic Value index for this, which resolves site through getQueryableValue() and stores a Site object for every submission.

The index is persisted to the Laravel cache. When cache.serializable_classes is restricted (newer Laravel skeletons default it to false), Site isn't on the allowlist, so on any request that reads the cached index the values come back as __PHP_Incomplete_Class and the query throws:

strtolower(): Argument #1 ($string) must be of type string, __PHP_Incomplete_Class given

It's intermittent because the request that builds the index works fine; only subsequent cached reads fail.

This adds a dedicated site store index for submissions that stores the site handle, matching how entries and terms index their site. Querying by a Site object still works since comparisons cast it to its handle.

🤖 Generated with Claude Code

[jasonvarga]

Superseded by #15641, which fixes this in 6.x by making Site a QueryableValue.