I'am experiencing the exact same issue, so input about this will be useful for me as well.
This is actually the JBoss Tools JAX-RS plugin that gives you this error message. It did not recognize your sublass of RESTEasy's PreProcessInterceptor as a valid JAX-RS Provider, because this class is an not part of the JAX-RS 1.1 spec.
Nonetheless, you get rid of those false-positive message by disabling this specific validation. Go to Workbench Preferences>JBoss Tools>JAX-RS>JAX-RS Validator, then set the severity level to "Ignore" for the "Missing Provider Implementation" rule in the "JAX-RS Providers" category. You can also configure this rule at the project level if you prefer.
I opened [JBIDE-16279] Support implementation specific providers - JBoss Issue Tracker to fix that problem in our tooling.