fix(http): fix setHeaders throwing ERR_HTTP_HEADERS_SENT on new requests

The `setHeaders` method used `this[headerStateSymbol] !== NodeHTTPHeaderState.none`
which threw when `headerStateSymbol` was undefined (ClientRequest never calls the
OutgoingMessage constructor) and was also stricter than Node.js. Align the check
with `setHeader` (singular) by only throwing when headers have actually been sent.

Closes #27049

Co-Authored-By: Claude <noreply@anthropic.com>

src/js/node/_http_outgoing.ts

@@ -271,7 +271,7 @@ const OutgoingMessagePrototype = {
     return this;
   },
   setHeaders(headers) {
-    if (this._header || this[headerStateSymbol] !== NodeHTTPHeaderState.none) {
+    if ((this._header !== undefined && this._header !== null) || this[headerStateSymbol] == NodeHTTPHeaderState.sent) {
       throw $ERR_HTTP_HEADERS_SENT("set");
     }
 

test/regression/issue/27049.test.ts

@@ -0,0 +1,85 @@
+import { expect, test } from "bun:test";
+import http from "node:http";
+
+test("ClientRequest.setHeaders should not throw ERR_HTTP_HEADERS_SENT on new request", async () => {
+  await using server = Bun.serve({
+    port: 0,
+    fetch(req) {
+      return new Response(req.headers.get("x-test") ?? "missing");
+    },
+  });
+
+  const { resolve, reject, promise } = Promise.withResolvers<string>();
+
+  const req = http.request(`http://localhost:${server.port}/test`, { method: "GET" }, res => {
+    let data = "";
+    res.on("data", (chunk: Buffer) => {
+      data += chunk.toString();
+    });
+    res.on("end", () => resolve(data));
+  });
+
+  req.on("error", reject);
+
+  // This should not throw - headers haven't been sent yet
+  req.setHeaders(new Headers({ "x-test": "value" }));
+
+  req.end();
+
+  const body = await promise;
+  expect(body).toBe("value");
+});
+
+test("ClientRequest.setHeaders works with Map", async () => {
+  await using server = Bun.serve({
+    port: 0,
+    fetch(req) {
+      return new Response(req.headers.get("x-map-test") ?? "missing");
+    },
+  });
+
+  const { resolve, reject, promise } = Promise.withResolvers<string>();
+
+  const req = http.request(`http://localhost:${server.port}/test`, { method: "GET" }, res => {
+    let data = "";
+    res.on("data", (chunk: Buffer) => {
+      data += chunk.toString();
+    });
+    res.on("end", () => resolve(data));
+  });
+
+  req.on("error", reject);
+
+  req.setHeaders(new Map([["x-map-test", "map-value"]]));
+
+  req.end();
+
+  const body = await promise;
+  expect(body).toBe("map-value");
+});
+
+test("ServerResponse.setHeaders should not throw before headers are sent", async () => {
+  const { resolve, reject, promise } = Promise.withResolvers<string>();
+
+  const server = http.createServer((req, res) => {
+    // This should not throw - headers haven't been sent yet
+    res.setHeaders(new Headers({ "x-custom": "server-value" }));
+    res.writeHead(200);
+    res.end("ok");
+  });
+
+  server.listen(0, () => {
+    const port = (server.address() as any).port;
+    const req = http.request(`http://localhost:${port}/test`, res => {
+      resolve(res.headers["x-custom"] as string);
+      server.close();
+    });
+    req.on("error", e => {
+      server.close();
+      reject(e);
+    });
+    req.end();
+  });
+
+  expect(await promise).toBe("server-value");
+});