Fix Rack websocket upgrades for 0.2.27

[wouterken]

What changed

Fix Rack websocket upgrade handling for callable streaming bodies that return 101 Switching Protocols, and add a regression test using Async::WebSocket::Adapters::Rack under Rack::Lint.

Why it changed

Issue #25 exposed that Itsi handled upgrade responses correctly for explicit rack.hijack callbacks, but not for Rack apps that expose the upgrade path via a callable response body. That caused websocket adapters to receive an Itsi::HttpResponse instead of a bidirectional IO stream and fail during frame reads.

Impact

This makes websocket upgrade behavior conform better to modern Rack behavior and fixes the async websocket compatibility regression targeted for 0.2.27.

Root cause

For generic streaming bodies, rack_interface.rb always yielded the HTTP response writer. For 101 upgrade responses it must instead yield the partial hijack stream so the Rack adapter gets real bidirectional IO.

Validation

• bundle exec ruby -Ilib:test test/rack/test_rack_server.rb in gems/server
• bundle exec rake test in gems/server
• root bundle exec rake test

Notes

This PR also suppresses expected bridge shutdown noise (BrokenPipe, reset/EOF-style shutdown cases) so normal websocket close paths do not emit misleading errors.

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