Expose HTTP protocol names

[samuel-williams-shopify]

Summary

• Expose the complete set of protocol names supported by the plaintext HTTP negotiator.
• Preserve the configured HTTP/2-then-HTTP/1 preference order while removing duplicate names.
• Cover both the default negotiator and a restricted protocol configuration.

This makes Async::HTTP::Protocol::HTTP.names consistent with the existing HTTPS.names interface and allows integrations to serialize protocol capabilities without depending on Async::HTTP implementation constants.

Testing

• bundle exec sus test/async/http/protocol/http.rb (7 tests, 9 assertions)
• bundle exec bake test (229 passed, 3 skipped, 33,904 assertions)
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib (292/292 definitions)