chore(tombstone-library): generate Rakefile for release builds

[torreypayne]

When tombstoning a library using toys tombstone-library, all existing files in the gem directory are purged and replaced with a minimal tombstone skeleton (README.md, LICENSE.md, .gemspec, .yardopts, version.rb). However, the tool previously omitted a Rakefile, leading to errors when releasing the tombstoned version of the package.

Context & Failure Mode

During the recent tombstoning of the stackdriver gem (googleapis/google-cloud-ruby#34614), automated release packaging via release-please (triggered on tag stackdriver/v1.0.0) failed in the release CI/CD workflow with:

rake aborted!
Don't know how to build task 'build' (See the list of available tasks with `rake --tasks`)

Because release packaging automation relies on executing bundle exec rake build to compile the .gem artifact prior to RubyGems publication, the absence of a Rakefile invoking bundler/gem_tasks aborted the release pipeline. We had to manually push a follow-up patch PR (googleapis/google-cloud-ruby#34652) to restore a minimal Rakefile, which caused release tag collisions and required manual changelog fixes.

Changes

• Adds toys/tombstone-library/.data/rakefile-template.erb containing the standard license header, require "bundler/setup", and require "bundler/gem_tasks".
• Updates generate_files in toys/tombstone-library/.toys.rb to generate Rakefile automatically when tombstoning any library.

Downstream Reference

Tested and validated downstream against the Apigee Registry tombstone draft PR: googleapis/google-cloud-ruby#36233.

[aandreassa]

All chores! LGTM