Add an experimental Rails main target to the CI matrix

gemfiles/8.2.gemfile tracks the rails/rails main branch (8.2.0.alpha), so
upstream removals surface here before they reach a stable release. The matrix
entry runs on Ruby 3.4 because Rails main requires Ruby >= 3.3.1.

The job is flagged experimental and runs under continue-on-error so that
in-flight upstream changes cannot turn the matrix red, and it skips
bundler-cache: a cached lockfile would pin the git source to a stale revision
instead of resolving the current branch head.