build: support ruby 3.1 and modern libpulsar

[gmiklos-ltg]

Summary

• add Ruby 3.1 and old Rake compatibility shims so the native build runs again
• update libpulsar integration for modern 3.7.x and 4.2.x style APIs plus cross-platform discovery
• add repo-local mise patching for Ruby 3.1.7 on current macOS and document Linux Docker verification

Test Plan

• mise exec -- bundle 1.17.3 exec rspec spec/pulsar/client_configuration_spec.rb
• mise exec -- bundle 1.17.3 exec rake compile
• mise exec -- bundle 1.17.3 exec ruby -e 'require "pulsar/client"; puts Pulsar::Client::VERSION'
• docker run --rm --entrypoint /bin/bash -v "/Users/gabriel.miklos/Desktop/external-to-voyager/pulsar-client-ruby:/workspace" -w /workspace 127178877223.dkr.ecr.us-east-2.amazonaws.com/learn-test/learn-base-image:1774966705 -lc 'bundle install && bundle exec rake compile && bundle exec ruby -e "require "pulsar/client"; puts Pulsar::Client::VERSION"'