feat(FN-5601): add OpenAI Responses API type support to custom providers

Added OpenAI Responses API as a new custom provider type, wiring `apiType: "responses"` through the core registry, engine routes, and dashboard UI with a dropdown selector; includes test coverage across the registry, routes, and component layers.

Fusion-Task-Id: FN-5601

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5601

packages/engine/src/__tests__/custom-providers-openai-completions.test.ts

@@ -0,0 +1,69 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { AuthStorage, ModelRegistry } from "@mariozechner/pi-coding-agent";
+import { completeSimple } from "@mariozechner/pi-ai";
+import { customProviderRegistryKey, type CustomProvider } from "@fusion/core";
+
+describe("custom providers openai-completions regression", () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("registers under slug key and completes a chat round-trip", async () => {
+    const authStorage = AuthStorage.inMemory();
+    const modelRegistry = ModelRegistry.inMemory(authStorage);
+
+    const providers: CustomProvider[] = [
+      {
+        id: "550e8400-e29b-41d4-a716-446655440000",
+        name: "My AI Provider",
+        apiType: "openai-compatible",
+        baseUrl: "https://example.test/v1",
+        apiKey: "CUSTOM_KEY",
+        models: [{ id: "my-model", name: "My Model" }],
+      },
+    ];
+
+    const provider = providers[0]!;
+    modelRegistry.registerProvider(customProviderRegistryKey(provider, providers), {
+      baseUrl: provider.baseUrl,
+      api: "openai-completions",
+      apiKey: provider.apiKey,
+      models: [{
+        id: "my-model",
+        name: "My Model",
+        reasoning: false,
+        input: ["text"],
+        cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+        contextWindow: 128000,
+        maxTokens: 16384,
+      }],
+    });
+    modelRegistry.refresh();
+
+    const registered = modelRegistry.getAll().find((model) => model.id === "my-model");
+    expect(registered?.provider).toBe("my-ai-provider");
+
+    vi.stubGlobal("fetch", vi.fn(async () => {
+      const stream = new ReadableStream({
+        start(controller) {
+          controller.enqueue(new TextEncoder().encode("data: {\"id\":\"chatcmpl-test\",\"object\":\"chat.completion.chunk\",\"choices\":[{\"index\":0,\"delta\":{\"role\":\"assistant\",\"content\":\"Hello from mock transport\"},\"finish_reason\":null}]}\n\n"));
+          controller.enqueue(new TextEncoder().encode("data: {\"id\":\"chatcmpl-test\",\"object\":\"chat.completion.chunk\",\"choices\":[{\"index\":0,\"delta\":{},\"finish_reason\":\"stop\"}],\"usage\":{\"prompt_tokens\":1,\"completion_tokens\":1,\"total_tokens\":2}}\n\n"));
+          controller.enqueue(new TextEncoder().encode("data: [DONE]\n\n"));
+          controller.close();
+        },
+      });
+
+      return new Response(stream, {
+        status: 200,
+        headers: { "content-type": "text/event-stream" },
+      });
+    }));
+
+    const model = modelRegistry.find("my-ai-provider", "my-model");
+    expect(model).toBeDefined();
+    const response = await completeSimple(model!, {
+      messages: [{ role: "user", content: "Hi", timestamp: Date.now() }],
+    });
+    expect(response.role).toBe("assistant");
+  });
+});

packages/engine/src/__tests__/pi-create-fn-agent.test.ts

@@ -1124,13 +1124,21 @@ describe("createFnAgent", () => {
   it("registers custom providers from global settings", async () => {
     readCustomProvidersMock.mockReturnValue([
       {
-        id: "custom-openai",
+        id: "550e8400-e29b-41d4-a716-446655440000",
         name: "Custom OpenAI",
         apiType: "openai-compatible",
         baseUrl: "https://custom.example/v1",
         apiKey: "CUSTOM_API_KEY",
         models: [{ id: "custom-model", name: "Custom Model" }],
       },
+      {
+        id: "660e8400-e29b-41d4-a716-446655440001",
+        name: "Custom Responses",
+        apiType: "openai-responses",
+        baseUrl: "https://responses.example/v1",
+        apiKey: "RESPONSES_API_KEY",
+        models: [{ id: "responses-model", name: "Responses Model" }],
+      },
     ] as any);
 
     const { createFnAgent } = await import("../pi.js");
@@ -1149,6 +1157,12 @@ describe("createFnAgent", () => {
       apiKey: "CUSTOM_API_KEY",
       models: [expect.objectContaining({ id: "custom-model", name: "Custom Model" })],
     }));
+    expect(registerProviderMock).toHaveBeenCalledWith("custom-responses", expect.objectContaining({
+      baseUrl: "https://responses.example/v1",
+      api: "openai-responses",
+      apiKey: "RESPONSES_API_KEY",
+      models: [expect.objectContaining({ id: "responses-model", name: "Responses Model" })],
+    }));
   });
 
   it("avoids lock-based SettingsManager.create when loading extension providers", async () => {