Fix Rails Main workflow Ruby version mismatch

Remove Gemfile.lock before bundle install to avoid Ruby version
conflicts when testing with Ruby 4.0.1.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ndbroadbent

.github/workflows/rails-main.yml

@@ -12,6 +12,9 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Remove Gemfile.lock for variable Ruby/Rails versions
+        run: rm -f Gemfile.lock
+
       - name: Setup Ruby 4.0.1
         uses: ruby/setup-ruby@v1
         with: