FEATURE: Validate tags in WatchedWords (#17254)

* FEATURE: Validate tags in WatchedWords

We didn't validate watched words automatic tagging, so it was possible
for an admin to created watched words with an empty tag list which would
result in an exception when users tried to create a new topic that
matched the misconfigured watched word.

Bug report: https://meta.discourse.org/t/lib-topic-creator-fails-when-the-word-math-appears-in-the-topic-title-or-text/231018?u=falco

spec/models/watched_word_spec.rb

@@ -89,6 +89,12 @@ describe WatchedWord do
       }.to_not change { described_class.count }
     end
 
+    it "error when an tag action is created without valid tags" do
+      expect {
+        described_class.create!(word: "ramones", action: described_class.actions[:tag])
+      }.to raise_error(ActiveRecord::RecordInvalid)
+    end
+
     it "replaces link with absolute URL" do
       word = Fabricate(:watched_word, action: described_class.actions[:link], word: "meta1")
       expect(word.replacement).to eq("http://test.localhost/")