fix: Use top level namespaces

When used in conjunction with the `OpenTelemetry::Instrumenation::ActiveSupport`,
the classloader would mistakenly use the wrong namespace and raises a `NameError`.

This change updates references to ensure we use top level namespaces to load the appropriate classes.

instrumentation/active_job/lib/opentelemetry/instrumentation/active_job/handlers.rb

@@ -57,7 +57,7 @@ def subscribe
           }
 
           @subscriptions = handlers_by_pattern.map do |key, handler|
-            ActiveSupport::Notifications.subscribe("#{key}.active_job", handler)
+            ::ActiveSupport::Notifications.subscribe("#{key}.active_job", handler)
           end
         end
 

instrumentation/active_job/lib/opentelemetry/instrumentation/active_job/instrumentation.rb

@@ -17,7 +17,7 @@ class Instrumentation < OpenTelemetry::Instrumentation::Base
         end
 
         present do
-          defined?(::ActiveJob)
+          defined?(::ActiveJob) && defined?(::ActiveSupport)
         end
 
         compatible do