FIX: Handle deprecations correctly in server-side pretty-text (#25059)

`window.deprecationWorkflow` does not exist in the server-side pretty-text environment. This commit fixes the check and adds a general spec for deprecations triggered inside pretty-text

spec/lib/pretty_text_spec.rb

@@ -2681,4 +2681,16 @@ HTML
 
     expect(cooked.strip).to eq(html.strip)
   end
+
+  it "handles deprecations correctly" do
+    Rails
+      .logger
+      .expects(:warn)
+      .once
+      .with("[PrettyText] Deprecation notice: Some deprecation message")
+
+    PrettyText.v8.eval <<~JS
+      require("discourse-common/lib/deprecated").default("Some deprecation message");
+    JS
+  end
 end