Implement UrlValidator plugin to flag invalid URLs

Implement a UrlValidator plugin to warn (or reject) requests with invalid commands/values.

Start by locating the plugin entry points and request-handling code for ImageResizer, then determine how existing plugins report validation problems. The issue does not define which commands or values are invalid, or whether warning and rejection are both required; completion would need those rules implemented and covered by the project's validation tests.

Written by the indexing model from the issue text.