undici transport: native header rejections, fileBody short-write detection, SOCKS proxy type #81

Part of the audit remediation umbrella #67. Milestone 5. Severity: HIGH (first item), MEDIUM (rest). Requirements: TRANSPORT-11, TRANSPORT-12, TRANSPORT-30, BODY-13.

Current SDK behavior

• Native header rejections escape send(). UNDICI_FORBIDDEN_HEADERS
  (packages/transport-undici/src/undici-transport.ts:58-62) drops content-length, host,
  transfer-encoding only. undici 6.28.0 (lib/core/request.js:397-409) throws
  InvalidArgumentError for keep-alive and upgrade, NotSupportedError for expect, and
  InvalidArgumentError for connection with a value other than close / keep-alive.
  toDispatchError (:221-231) maps these to a bare non-retryable TypeError; nothing is
  sent. toUndiciHeaders (:150-160) has no per-header guard, so a model-valid non-token name
  such as X Custom (core admits printable non-token bytes, ascii-validation.ts:29-33) is
  rejected by undici's isValidHTTPToken and the send fails. fetch-transport.ts:146-152 wraps
  each native.append in a try/catch and degrades to a drop; undici has no equivalent. No
  test exercises a non-token header name.
• fileBody bypasses short-write detection. The file-body branch (:259-273) hands
  createReadStream(path, {start, end}) to undici directly. writeTo and its
  transferred === count invariant (packages/body-file/src/file-body.ts:75-78) never run.
  content-length is dropped, so undici frames the body chunked and the wire cannot detect a
  short write either. A 1 MB fileBody truncated to 10 bytes before send(): undici POSTs 10
  bytes and resolves 200; fetch raises TransportFailureError. The Phase 8a checklist (:80)
  marks BODY-13 done while recording the bypass.
• A SOCKS proxy type throws undici's raw error. toProxyAgentOptions (:115-122) uses
  proxy.type as the URI scheme with no check. Core resolves ALL_PROXY=socks5://host:1080 to
  type: 'socks5' (config/proxy.ts:34,372-380). new ProxyAgent({uri: 'socks5://…'}) throws
  InvalidArgumentError('Invalid URL protocol: …') out of the public factory, untyped and
  undocumented.

Expected behavior

undiciTransport().send() does not throw a native error for any model-valid request. A
header the native layer cannot carry is dropped and logged, the same as in the fetch
transport. A file body that ends short of its declared length fails the send with a
TransportFailureError naming transferred-of-total on both transports. undiciTransport()
rejects an unsupported proxy type with a typed, documented error.

Notes and leads

• Drop set: add expect, keep-alive, upgrade; drop connection values other than
  close / keep-alive. Validate each name against RFC 9110 token in toUndiciHeaders and
  degrade to a logged drop. Check FETCH_FORBIDDEN_HEADERS (fetch-transport.ts:45-50) too:
  on Node the global fetch is undici-backed and its Headers does not implement forbidden
  names, so the same rejections likely surface there as a retryable TransportFailureError.
• File body: route through writeTo via the pumpBody path the streamed case uses, or wrap
  the read stream to count bytes and destroy with TransportFailureError when it ends short.
• Proxy: reject proxy.type !== 'http' with a typed DexpaceError (or the documented
  TypeError), add @throws, and either record "SOCKS resolved by core, supported by neither
  transport" in docs/deviations.md or drop socks* from ProxyType before the first bump.
• Every item gets a row in packages/transport-conformance that both transports run:
  Expect: 100-continue, Upgrade: websocket, a non-token name, truncate-after-stat.
• Patch changesets for @dexpace/transport-undici, @dexpace/transport-fetch, and
  @dexpace/core if ProxyType changes.