quic: emit RFC 9114 stream-closure error codes (H3_REQUEST_CANCELLED, H3_REQUEST_INCOMPLETE)

Following #65442, which made rejected request streams reset with H3_REQUEST_REJECTED, two more RFC 9114 stream-closure codes are still not emitted. Filing to agree on direction before implementing — one needs a small API decision.

1. Cancellation — H3_REQUEST_CANCELLED (0x10c)

When an endpoint gives up on a request it no longer wants (client aborting a request, or a server abandoning a response after starting it), the stream should be reset as cancelled. There are two gaps today:

• A cancel is two operations, with no single one. Fully cancelling a bidirectional stream means aborting both halves — RESET_STREAM ("I will not send more") and STOP_SENDING ("do not send me more").
• The code is wrong. Both default to 0n (H3_NO_ERROR), so a cancel currently signals "no error" to the peer.

// Today: two calls, and both default to H3_NO_ERROR (0) — the peer is told "no error"
stream.resetStream();
stream.stopSending();

RFC 9114 §4.1.1: "Client SHOULD use the error code H3_REQUEST_CANCELLED to cancel requests", and a server abandoning a response after partial processing "SHOULD abort its response stream with the error code H3_REQUEST_CANCELLED." node neither defaults to 0x10c nor exposes it.

Proposal: a stream.cancel([reason]) method that resets both halves with H3_REQUEST_CANCELLED. Role-agnostic (client-cancel and server-abandon use the same code); the existing resetStream()/stopSending() stay as low-level primitives.

// Proposed: one call, resets both halves with H3_REQUEST_CANCELLED (0x10c)
stream.cancel();

This keeps the numeric code out of consumer code (e.g. a future h3 client in undici, #5471), matching how node:http2 offers stream.close(code) and how fetch/AbortController sits a layer above.

Alternative considered: expose a named H3_REQUEST_CANCELLED constant for the existing resets. Still two calls, and it pushes the code choice onto every caller:

// Alternative: still two calls, caller supplies the code via an exposed constant
stream.resetStream(<QUIC_CONSTANTS>.H3_REQUEST_CANCELLED);
stream.stopSending(<QUIC_CONSTANTS>.H3_REQUEST_CANCELLED);

2. Incomplete request — H3_REQUEST_INCOMPLETE (0x10d)

RFC 9114 §4.1: "If a client-initiated stream terminates without enough of the HTTP message to provide a complete response, the server SHOULD abort its response stream with the error code H3_REQUEST_INCOMPLETE."

This needs no API — the HTTP/3 layer can emit it when a request stream closes before a complete message. Today the teardown uses a generic code (H3_NO_ERROR or H3_INTERNAL_ERROR), so the client can't distinguish "the server errored" from "my request was incomplete." Emitting 0x10d puts the signal where it belongs.

Issue に記載されている QUIC HTTP/3 ストリームの teardown パスから始め、RFC 9114 のセクション 4.1 と 4.1.1 を確認してください。まず、統合された stream.cancel([reason]) API が必要かどうかを判断し、次にキャンセルで H3_REQUEST_CANCELLED が使用され、不完全なクライアントリクエストストリームで H3_REQUEST_INCOMPLETE が使用されることを確認してください。両方の動作について coverage を追加または更新してください。