improve(node:http): uncork after flushing headers to ensure data is sent immediately (#23413)

### What does this PR do?
Calls `uncork()` after flushing response headers to ensure data is sent
as soon as possible, improving responsiveness.
This behavior still works correctly even without the explicit `uncork()`
call, due to the deferred uncork logic implemented here:

6e3359dd16/packages/bun-uws/src/Loop.h (L57-L64)

A test already covers this scenario in
`test/js/node/test/parallel/test-http-flush-response-headers.js`.


### How did you verify your code works?
CI

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

packages/bun-uws/src/HttpResponse.h

@@ -478,7 +478,7 @@ public:
         return internalEnd({nullptr, 0}, 0, false, false, closeConnection);
     }
 
-    void flushHeaders() {
+    void flushHeaders(bool flushImmediately = false) {
 
         writeStatus(HTTP_200_OK);
 
@@ -499,6 +499,10 @@ public:
             Super::write("\r\n", 2);
             httpResponseData->state |= HttpResponseData<SSL>::HTTP_WRITE_CALLED;
         }
+        if (flushImmediately) {
+            /* Uncork the socket to send data to the client immediately */
+            this->uncork();
+        }
     }
     /* Write parts of the response in chunking fashion. Starts timeout if failed. */
     bool write(std::string_view data, size_t *writtenPtr = nullptr) {