Preserve streaming body terminal errors

[samuel-williams-shopify]

Summary

• preserve queued chunks before surfacing producer-side terminal errors
• make body completion first-terminal-result-wins so later closure cannot poison clean EOF
• keep IO#close zero-argument while using an explicit close_with_error(error) capability for error-aware body streams
• detach Reader bodies after failed consumption and add lifecycle regression coverage

This establishes the transport-neutral body semantics needed to distinguish HTTP/2 END_STREAM completion from RST_STREAM cancellation in HTTP/1 <-> HTTP/2 proxies. The explicit error-close capability avoids method-arity probing and prevents a streaming failure from being translated into a clean HTTP/2 END_STREAM.

Testing

• full protocol-http suite: 780 tests, 1,584 assertions
• RuboCop on all changed files
• documentation coverage: 470/470 public definitions
• git diff --check