Add one model host per provider and the unwrapping walk

app/MindWork AI Studio/Models/Hosting/Hosts/HostOpenAI.cs

@@ -0,0 +1,28 @@
+using AIStudio.Provider;
+
+namespace AIStudio.Models.Hosting.Hosts;
+
+/// <summary>
+/// OpenAI's own cloud, the one place where the Responses API is actually spoken.
+/// </summary>
+/// <remarks>
+/// This is the single host that does not put its models on the ordinary chat completion API,
+/// because it is the single place the app sends a Responses API request from. Everywhere else a
+/// GPT model is reached -- a gateway, a reseller, somebody's own proxy -- it is reached through the
+/// ordinary API, and the host there says so.
+/// </remarks>
+public sealed class HostOpenAI : ModelHost
+{
+    /// <inheritdoc />
+    public override LLMProviders Provider => LLMProviders.OPEN_AI;
+
+    /// <inheritdoc />
+    public override ModelSource Source => new("https://platform.openai.com/docs/api-reference/responses", new DateOnly(2026, 9, 11), "Models are named plainly, and both the Responses API and the chat completion API are served here.");
+
+    /// <inheritdoc />
+    /// <remarks>
+    /// Nothing is taken away: whichever of the two APIs a model states, it can be reached through
+    /// it here.
+    /// </remarks>
+    public override ModelProfile ApplyTransport(in ModelProfile profile) => profile;
+}