Add concrete Langfuse tracer and prompt gateway

test/core/langfuse/LangfuseTracer.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it, vi } from 'vitest';
+
+const generationEnd = vi.fn();
+const generation = vi.fn().mockReturnValue({ end: generationEnd });
+const trace = vi.fn().mockReturnValue({ id: 'lf-trace-1', generation });
+const flushAsync = vi.fn().mockResolvedValue(undefined);
+
+vi.mock('langfuse', () => ({
+  Langfuse: vi.fn().mockImplementation(() => ({ trace, flushAsync })),
+}));
+
+import { LangfuseTracer } from '../../../src/core/langfuse/LangfuseTracer';
+
+describe('LangfuseTracer', () => {
+  it('starts a Langfuse trace and exposes its id', () => {
+    const tracer = new LangfuseTracer({ publicKey: 'pk', secretKey: 'sk', baseUrl: 'http://lf' });
+    const handle = tracer.startTrace('my-pipeline', { foo: 'bar' });
+
+    expect(trace).toHaveBeenCalledWith({ name: 'my-pipeline', metadata: { foo: 'bar' } });
+    expect(handle.id).toBe('lf-trace-1');
+  });
+
+  it('startGeneration creates a Langfuse generation and end() forwards output/usage', () => {
+    const tracer = new LangfuseTracer({ publicKey: 'pk', secretKey: 'sk', baseUrl: 'http://lf' });
+    const handle = tracer.startTrace('my-pipeline');
+    const gen = handle.startGeneration({ name: 'step-1', model: 'llama3', input: ['hi'] });
+    gen.end({ output: 'done', usage: { totalTokens: 5 } });
+
+    expect(generation).toHaveBeenCalledWith(
+      expect.objectContaining({ name: 'step-1', model: 'llama3', input: ['hi'] })
+    );
+    expect(generationEnd).toHaveBeenCalledWith(
+      expect.objectContaining({ output: 'done', usage: { totalTokens: 5 } })
+    );
+  });
+
+  it('flush() calls flushAsync on the underlying client', async () => {
+    const tracer = new LangfuseTracer({ publicKey: 'pk', secretKey: 'sk', baseUrl: 'http://lf' });
+    await tracer.flush();
+    expect(flushAsync).toHaveBeenCalledTimes(1);
+  });
+});