Unbreak support for v4 input file.

Reverts part of 6e828eb but adds to test coverage now that I
understand the other changes v4 requires.

Author: flavorjones

lib/tailwindcss/commands.rb

@@ -10,12 +10,12 @@ def tailwindcss_version
       def compile_command(debug: false, **kwargs)
         command = [
           Tailwindcss::Ruby.executable(**kwargs),
-          "-o", Rails.root.join("app/assets/builds/tailwind.css").to_s
+          "-o", Rails.root.join("app/assets/builds/tailwind.css").to_s,
+          "-i", Rails.root.join("app/assets/stylesheets/application.tailwind.css").to_s,
         ]
 
         unless tailwindcss_version >= "4.0"
           command += [
-            "-i", Rails.root.join("app/assets/stylesheets/application.tailwind.css").to_s,
             "-c", Rails.root.join("config/tailwind.config.js").to_s,
           ]
         end

test/integration/user_journey_test.sh

@@ -32,6 +32,12 @@ bundle install
 bundle show --paths
 bundle binstubs --all
 
+if bundle show | fgrep tailwindcss-ruby | fgrep -q "(4." ; then
+  TAILWIND4=1
+else
+  TAILWIND4=0
+fi
+
 # install tailwindcss
 bin/rails tailwindcss:install
 
@@ -45,6 +51,13 @@ task :still_here do
 end
 EOF
 
+if [[ $TAILWIND4 = 1 ]] ; then
+  cat > app/assets/stylesheets/application.tailwind.css <<EOF
+@import "tailwindcss";
+@theme { --color-special: #abc12399; }
+EOF
+fi
+
 bin/rails tailwindcss:build still_here | grep "Rake process did not exit early"
 
 if [[ $(rails -v) > "Rails 8.0.0.beta" ]] ; then
@@ -61,4 +74,9 @@ grep -q "Show this post" app/views/posts/index.html.erb
 bin/rails tailwindcss:build[verbose]
 grep -q "py-2" app/assets/builds/tailwind.css
 
+if [[ $TAILWIND4 = 1 ]] ; then
+  # TEST: contents include application.tailwind.css
+  grep -q "#abc12399" app/assets/builds/tailwind.css
+fi
+
 echo "OK"