fix: align parseCookieHeader return type with getAll cookie method

[TheSeydiCharyyev]

Description

parseCookieHeader returns { name: string; value?: string }[], but the getAll cookie method (GetAllCookies in CookieMethodsServer) requires { name: string; value: string }[]. As a result, the documented server pattern fails to type-check:

getAll() {
  return parseCookieHeader(request.headers.get("cookie") ?? "");
  // Type '{ name: string; value?: string }[]' is not assignable to
  // '{ name: string; value: string }[]'.
}

Fix

Normalize the value with ?? "" and drop the optional from the return type, so parseCookieHeader is directly assignable to getAll. This mirrors the library's own documentCookieGetAll, which already uses parsed[name] ?? "".

For a key returned by Object.keys(parsed) the value is always a string at runtime, so this is a no-op for normal cookies and normalizes a valueless cookie (a=) to "" — consistent with documentCookieGetAll. Narrowing the return type from string | undefined to string is not a breaking change for consumers reading the value.

Testing

• Added a regression test in helpers.spec.ts that asserts the returned values are strings and that parseCookieHeader satisfies the GetAllCookies contract at compile time (const getAll: GetAllCookies = () => parseCookieHeader("a=b")), so a future regression fails tsc.
• pnpm build (tsc), pnpm test (87 passed), pnpm exec prettier --check, pnpm exec eslint — all pass. Existing snapshots unchanged.

Fixes #115