fix: preserve Cookie headers in extension requests (#1581)

pkg/download/engine/testdata/wpt/UPSTREAM.md

@@ -9,7 +9,7 @@ The files under `fetch/` are copied from the official
 
 Gopeed executes applicable worker-style Fetch API tests at the extension
 JavaScript boundary. The checked-in runner currently executes unchanged WPT
-tests for Headers, no-CORS header guards, Request construction and body
+tests for Headers syntax and iteration, Request construction and body
 consumption, ReadableStream bodies, keepalive, Request errors, Response
 construction and body consumption, immutable network response headers, and
 the Response static constructors.
@@ -19,10 +19,34 @@ a claim that Gopeed is a browser. Browser-only suites that require a document,
 CORS origin enforcement or preflight, CSP, Mixed Content, Service Workers,
 browser HTTP cache, navigation, browser authentication UI, or WPT's
 multi-origin server infrastructure are intentionally excluded. Gopeed also
-keeps one extension-specific behavior: `redirect: "manual"` exposes the HTTP
+keeps extension-specific behavior: `redirect: "manual"` exposes the HTTP
 redirect response so download extensions can inspect `Location`, instead of
 returning a browser `opaqueredirect` response.
 
+Header permissions follow [Node/Undici's server-side Fetch API](https://github.com/nodejs/undici/blob/main/README.md#forbidden-and-safelisted-header-names). Gopeed does not
+apply browser forbidden request/response header lists or no-CORS safelists.
+Extensions may supply authentication, referrer, origin, host, proxy and fetch
+metadata headers, and read `Set-Cookie` response headers. Network response
+headers remain immutable; invalid header syntax still raises an error.
+Repeated `Cookie` values are combined with `; `, as in Node/Undici.
+
+The runner excludes the browser-only `headers-no-cors.any.js` and
+`headers-forbidden-override.any.js` suites, the filtering assertions within
+`request-headers.any.js`, and the forbidden-response-cookie assertion within
+`header-setcookie.any.js`. Gopeed's `extension_headers_test.go` replaces those
+restrictions with integration tests that check headers actually received by
+an HTTP server through fetch and XMLHttpRequest, plus header mutations,
+cloning and response cookies. The upstream test files remain unchanged.
+
+Gopeed deliberately ignores transport controls that Node/Undici rejects:
+`Expect`, `Keep-Alive`, `Transfer-Encoding`, `Upgrade`, and `Connection` values
+other than `close` or `keep-alive`. `Content-Length` is generated from the
+actual request body, ignoring a supplied length. Explicit `Host` and
+`Sec-Fetch-Mode` values are preserved rather than replaced by defaults.
+These transport behaviors are also covered by integration tests.
+`credentials: "omit"` disables automatic CookieJar cookies but preserves
+explicit headers; Gopeed retains its extension CookieJar support.
+
 `wpt_harness.js` is a small Goja adapter maintained by Gopeed; it is not copied
 from WPT. Test logic below `fetch/` is unchanged; the checked-in copy may only
 normalize a final newline.