Add on_write_buffer_drained

tests/test_http_handler.cpp

@@ -88,3 +88,35 @@ TEST_CASE("HttpHandler route parsing edge cases") {
           HttpRoute::GET_retention);
   }
 }
+
+// Test helper to verify the new hook functionality
+struct MockConnectionHandler : public ConnectionHandler {
+  bool write_progress_called = false;
+  bool write_buffer_drained_called = false;
+
+  void on_write_progress(std::unique_ptr<Connection> &) override {
+    write_progress_called = true;
+  }
+
+  void on_write_buffer_drained(std::unique_ptr<Connection> &) override {
+    write_buffer_drained_called = true;
+  }
+};
+
+TEST_CASE("ConnectionHandler hooks") {
+  SUBCASE("on_write_buffer_drained hook exists") {
+    MockConnectionHandler handler;
+
+    // Verify hooks are available and can be overridden
+    CHECK_FALSE(handler.write_progress_called);
+    CHECK_FALSE(handler.write_buffer_drained_called);
+
+    // Would normally be called by Server during write operations
+    std::unique_ptr<Connection> null_conn;
+    handler.on_write_progress(null_conn);
+    handler.on_write_buffer_drained(null_conn);
+
+    CHECK(handler.write_progress_called);
+    CHECK(handler.write_buffer_drained_called);
+  }
+}