Fix flaky test TestHTTPConnectWithHeaders (#1397)

The test was failing intermittently due to a race condition where the
datasource connection attempt occurred before the proxy server was ready
to accept connections.

This fix adds synchronization logic that polls the proxy server port
until it's accepting connections (with a 5-second timeout)

Fixes #1397

Author: MattHandzel

pkg/plugin/driver_integration_test.go

@@ -1202,6 +1202,20 @@ func TestHTTPConnectWithHeaders(t *testing.T) {
 		assert.Equal(t, nil, err)
 	}()
 
+	// Wait for proxy server to be ready
+	proxyAddr := fmt.Sprintf("%s:%s", proxyHost, proxyPort)
+	for i := 0; i < 50; i++ {
+		conn, err := net.DialTimeout("tcp", proxyAddr, 100*time.Millisecond)
+		if err == nil {
+			conn.Close()
+			break
+		}
+		if i == 49 {
+			t.Fatalf("Proxy server failed to start after 5 seconds")
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+
 	username := getEnv("CLICKHOUSE_USERNAME", "default")
 	password := getEnv("CLICKHOUSE_PASSWORD", "")
 	secure := map[string]string{}