Preserve orderly HTTP/2 duplex shutdown.

[samuel-williams-shopify]

Align HTTP/2 application-facing input and output closure with HTTP/1 and duplex stream semantics.

• The input and output directions close independently. Ending local output does not implicitly cancel remote input.
• Closing an HTTP/2 input marks the application receive side as closed. Incoming DATA is discarded while restoring flow-control credit as long as local output remains active.
• Once both application-facing directions are closed, the remaining wire stream is terminated with RST_STREAM(NO_ERROR) rather than treating normal closure as cancellation.
• A received NO_ERROR reset is treated as orderly closure. Streamable bodies observe input EOF and unwind normally instead of having their task cancelled.
• Closing the owning HTTP/2 response remains an explicit fast cancellation and sends RST_STREAM(CANCEL) while its body is active. Input errors use INTERNAL_ERROR.
• Body::Pipe preserves the same independent-direction model: closing its write side closes the HTTP output without opportunistically stopping HTTP input.

The tests cover these contracts at their semantic boundaries:

• Shared proxy behavior verifies under HTTP/1.0, HTTP/1.1, and HTTP/2 that closing the local peer finishes the request direction while an idle response remains active. If later response data cannot be forwarded to the closed peer, the response direction closes and the pooled resource is released.
• Shared streamable-body behavior verifies under HTTP/1 and HTTP/2 that an echo completes and the server streaming block exits normally when its peer closes. This captures the same lifecycle required by async-websocket without depending on WebSocket framing.
• HTTP/2-specific coverage confirms that closing input while output remains active discards incoming data without blocking the opposite direction.

Testing

• Full async-http suite: 264 passed, 3 skipped, 32,919 assertions.
• Focused proxy suite: 30 passed, 84 assertions.
• Focused pipe suite: 4 passed, 3 assertions.
• RuboCop over all tracked Ruby files: 120 files inspected, no offenses.
• git diff --check: clean.

Types of Changes

• Bug fix.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer Certificate of Origin 1.1.