chore: migrate last remaining tests of rspec

[seuros]

Found that has_closure_tree_root_spec.rb was already ported to Minitest
Ported matcher_spec.rb to test/closure_tree/matcher_test.rb
Found that migration_generator_spec.rb was already ported to Minitest

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[Copilot]

Pull Request Overview

This PR migrates the last remaining tests from RSpec to Minitest and removes all related RSpec support files and configurations. Key changes include:

• Changing require paths in test_helper.rb to reference test support files rather than spec support files.
• Adding a new Minitest test file (matcher_test.rb) for the closure_tree matcher while removing the corresponding RSpec tests.
• Removing obsolete support files, RSpec configuration (spec_helper.rb, .rspec), generator specs, and updates to the gemspec and Rakefile.

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/test_helper.rb	Updated require paths to load support files from test/support.
test/closure_tree/matcher_test.rb	New Minitest tests replacing the old RSpec matcher tests.
spec/support/*	Removed RSpec-specific helper files no longer needed.
spec/spec_helper.rb, migration_generator_spec.rb, spec/fixtures/tags.yml, spec/closure_tree/*	Entirely removed as part of migrating off RSpec.
closure_tree.gemspec	Removed RSpec development dependencies in favor of Minitest dependencies.
Rakefile	Removed RSpec tasks and updated the tasks to work with Minitest.
.rspec	Removed since it is no longer needed.