Add FakeRequest model to handle request data

Introduced a new `FakeRequest` class in the `ReC.API.Models` namespace.
The class includes `Body` and `Header` properties, both of which
are nullable dictionaries with string keys and object values.
These properties are marked as `init`-only to ensure immutability
after initialization. This model is designed to support flexible
handling of request payloads and headers.

src/ReC.API/Models/FakeRequest.cs

@@ -0,0 +1,8 @@
+namespace ReC.API.Models;
+
+public class FakeRequest
+{
+    public Dictionary<string, object>? Body { get; init; }
+
+    public Dictionary<string, object>? Header { get; init; }
+}