Support OpenRouter's real per-request cost reporting

Send usage: { include: true } on every chat completion request (an
OpenRouter-specific extension - requires Usage Accounting enabled on
the OpenRouter account; other OpenAI-compatible providers generally
ignore unknown top-level request fields). When the response includes
OpenRouter's own usage.cost, forward it to Langfuse as
costDetails.total, so cost is the real per-request figure OpenRouter
billed rather than a guess from Langfuse's model price list (which
has no entry for proxied/aggregated OpenRouter model names anyway).

Verified: full suite (84/84) passing, tsc/eslint clean, docker
compose build succeeds. Per Langfuse's OpenRouter integration guide:
https://langfuse.com/integrations/gateways/openrouter

test/core/llm/LlmClient.test.ts

@@ -29,6 +29,24 @@ describe('LlmClient', () => {
       temperature: 0.7,
       max_tokens: undefined,
       messages: [{ role: 'user', content: 'hi' }],
+      usage: { include: true },
+    });
+  });
+
+  it('extracts OpenRouter cost from the response usage when present', async () => {
+    createMock.mockResolvedValue({
+      choices: [{ message: { content: 'hello' } }],
+      usage: { prompt_tokens: 10, completion_tokens: 5, total_tokens: 15, cost: 0.00042 },
+    });
+
+    const client = new LlmClient({ baseUrl: 'https://openrouter.ai/api/v1', apiKey: 'key' });
+    const result = await client.chat([{ role: 'user', content: 'hi' }], { model: 'openrouter/auto' });
+
+    expect(result.usage).toEqual({
+      promptTokens: 10,
+      completionTokens: 5,
+      totalTokens: 15,
+      cost: 0.00042,
     });
   });