Use configurable generators for fixture cache pregeneration

[ngan]

Summary

• add a pluggable generator architecture for fixture cache pregeneration via FixtureKit.configuration.generator
• make FixtureKit::Generator a minimal base class that delegates class .run to instance #run
• add FixtureKit::TestCase::Generator as the default non-RSpec generator, running pregeneration inside an internal ActiveSupport::TestCase
• ensure the internal test-case harness is removed from Minitest::Runnable.runnables so pregeneration does not count toward suite stats
• keep RSpec integration on FixtureKit::RSpec::Generator so pregeneration runs in an internal RSpec example context with normal hooks/transactions and no reporter stat pollution
• add unit coverage for configuration defaults and generator harness behavior/failure cases
• update README docs for the new generator defaults and behavior

Testing

• bundle exec rspec --fail-fast
• BUNDLE_GEMFILE=gemfiles/rails_8.0.gemfile bundle exec rspec --fail-fast
• BUNDLE_GEMFILE=gemfiles/rails_8.1.gemfile bundle exec rspec --fail-fast