Ensure that upcasting only runs one extra time.

Problem/Motivation

As a follow-up to #2305199: Create a route for every variant and use a "route filter" to apply selection rules and a corollary to #2605250: ParamConverterManager::convert() should run once per request object, we can prevent upcasting from running too many times.

Proposed resolution

Subclass the ParamConversionEnhancer for now, until #2605250: ParamConverterManager::convert() should run once per request object goes in
Additionally, modify VariantRouteFilter so that it modifies the request if it finds a valid page manager route.

Remaining tasks

N/A

User interface changes

N/A

API changes

We now modify the global request during filtering

Data model changes

N/A