DomainSourceRouteMatcher caches the domain route key once per process, so cron writes other domains' resolved paths under the wrong domain (2.0.x backport)

Backport of !461 (merged) to 2.0.x.

Additive only, since this branch carries stable releases: a protected property, a public setter and a protected override. The static holding the route provider is deliberately left in place here, unlike on 3.x and 4.x. Removing it would drop a protected member from a class that is not marked internal, and it was a robustness improvement rather than part of the fix. The cache ID is resolved live in the provider either way.

The functional tests keep this branch's own service API; only the static resets are removed, since those resets are what hid the defect.

Unit tests run locally on core 11.3; functional coverage goes to CI.