Fix nil panics and route loss during config sync cycles

[kkincaidV]

Fix nil panics and route loss during config sync cycles

• provider.go: Return cached component when route file is deleted
  instead of returning nil. Prevents nil pointer panics on requests
  to routes whose YAML files were archived by the sync watcher.

• service.go: Skip router replacement when the new router has fewer
  routes than the existing one. Prevents file-consumption sync cycles
  from degrading a healthy router with a partial one.

• handler.go: Add nil guards for component and output type parameters
  in Handle and writeErrorResponse to prevent panics when a component
  is unavailable or partially loaded.