Add a validate_config for Naming/PredicateName

This will help make sure that users config makes sense and achieves the
behavior they intend it to, even if they don't read the docs.

This also eases the burden of the docs to mention this case so loudly.

changelog/new_config_validator_for_naming_predicate_name.md

@@ -0,0 +1 @@
+* [#13068](https://github.com/rubocop/rubocop/pull/13068): Add config validation to `Naming/PredicateName` to check that all `ForbiddenPrefixes` are being checked. ([@maxjacobson][])

lib/rubocop/cop/naming/predicate_name.rb

@@ -93,6 +93,16 @@ def on_def(node)
         end
         alias on_defs on_def
 
+        def validate_config
+          forbidden_prefixes.each do |forbidden_prefix|
+            next if predicate_prefixes.include?(forbidden_prefix)
+
+            raise ValidationError, <<~MSG.chomp
+              The `Naming/PredicateName` cop is misconfigured. Prefix #{forbidden_prefix} must be included in NamePrefix because it is included in ForbiddenPrefixes.
+            MSG
+          end
+        end
+
         private
 
         def allowed_method_name?(method_name, prefix)

spec/rubocop/cli_spec.rb

@@ -1013,6 +1013,27 @@ def meow_at_4am?
       end
     end
 
+    context 'when Naming/PredicateName has predicate prefix forbidden which is not listed in NamePrefix' do
+      it 'fails with an error message' do
+        create_file('example.rb', 'puts 1')
+        create_file('.rubocop.yml', <<~YAML)
+          Naming/PredicateName:
+            NamePrefix:
+              - is_
+              - has_
+            ForbiddenPrefixes:
+              - is_
+              - has_
+              - seems_to_be_
+        YAML
+        expect(cli.run(['example.rb'])).to eq(2)
+        expect($stderr.string.strip).to eq(
+          'Error: The `Naming/PredicateName` cop is misconfigured. Prefix seems_to_be_ must be ' \
+          'included in NamePrefix because it is included in ForbiddenPrefixes.'
+        )
+      end
+    end
+
     it 'allows the default configuration file as the -c argument' do
       create_file('example.rb', <<~RUBY)
         # frozen_string_literal: true