Fix ScriptHost resolution in ScriptRouteHandler

The ScriptHost is currently resolved using the ScriptHostManager static instance property. This should be improved to avoid many issues caused by that pattern today.

This work item has a dependency on the DI enhancements we'll be landing.

Locate ScriptRouteHandler and inspect how it currently obtains ScriptHost through the ScriptHostManager static instance property. Read the related DI enhancements when available and determine the intended injection boundary. Done means ScriptHost resolution no longer depends on that static instance and the existing handler behavior remains intact.

Written by the indexing model from the issue text.