fix(responseInterceptor): prevent trailer/content-length conflict

Causing this error to be thrown:

```shell
node:_http_outgoing:522
    throw new ERR_HTTP_TRAILER_INVALID();
    ^

Error [ERR_HTTP_TRAILER_INVALID]: Trailers are invalid with this transfer encoding
```

---------

Co-authored-by: zibo <ziboilihua@gmail.com>

Author: chimurai

src/handlers/response-interceptor.ts

@@ -74,6 +74,11 @@ export function responseInterceptor<
 
       // set correct content-length (with double byte character support)
       debug('set content-length: %s', Buffer.byteLength(interceptedBuffer));
+
+      // Buffered responses cannot preserve trailer framing.
+      // Remove trailer declaration (and transfer-encoding just in case) before setting content-length.
+      res.removeHeader('trailer');
+      res.removeHeader('transfer-encoding');
       res.setHeader('content-length', Buffer.byteLength(interceptedBuffer));
 
       debug('write intercepted response');
@@ -143,8 +148,10 @@ function copyHeaders<TRes extends http.ServerResponse = http.ServerResponse>(
   if (response.setHeader) {
     let keys = Object.keys(originalResponse.headers);
 
-    // ignore chunked, brotli, gzip, deflate headers
-    keys = keys.filter((key) => !['content-encoding', 'transfer-encoding'].includes(key));
+    // ignore encoding/framing headers that are incompatible with buffered interception
+    keys = keys.filter(
+      (key) => !['content-encoding', 'transfer-encoding', 'trailer'].includes(key),
+    );
 
     keys.forEach((key) => {
       let value = originalResponse.headers[key];

test/e2e/response-interceptor.spec.ts

@@ -157,4 +157,49 @@ describe('responseInterceptor()', () => {
       expect(response.body.deflated).toBe(true);
     });
   });
+
+  describe('trailer response header handling', () => {
+    beforeEach(async () => {
+      await targetServer
+        .forGet('/response-headers')
+        .withExactQuery('?Trailer=X-Stream-Error&Host=localhost')
+        .thenReply(200, '', {
+          'transfer-encoding': 'chunked',
+          trailer: 'X-Stream-Error',
+          host: 'localhost',
+          'set-cookie': 'cookie=monster; Path=/',
+          'content-type': 'application/json; charset=utf-8',
+          'cache-control': 'private, max-age=60',
+        });
+
+      agent = request(
+        createApp(
+          createProxyMiddleware({
+            target: targetServer.url,
+            changeOrigin: true,
+            selfHandleResponse: true,
+            on: {
+              proxyRes: responseInterceptor(async () => {
+                return JSON.stringify({ ok: true, note: 'rewritten' });
+              }),
+            },
+          }),
+        ),
+      );
+    });
+
+    it('should rewrite body without trailer/content-length conflict and preserve end-to-end headers', async () => {
+      const response = await agent
+        .get('/response-headers?Trailer=X-Stream-Error&Host=localhost')
+        .expect(200);
+
+      expect(response.body).toEqual({ ok: true, note: 'rewritten' });
+      expect(response.header['trailer']).toBeUndefined();
+      expect(response.header['transfer-encoding']).toBeUndefined();
+      expect(response.header['content-length']).toBeDefined();
+      expect(response.header['content-type']).toContain('application/json');
+      expect(response.header['set-cookie']).toBeDefined();
+      expect(response.header['cache-control']).toBe('private, max-age=60');
+    });
+  });
 });