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

src/core/langfuse/LangfuseTracer.ts

@@ -73,6 +73,7 @@ export class LangfuseTracer extends Tracer {
             generation.update({
               output: output.output,
               usageDetails: toUsageDetails(output.usage) as Record<string, number> | undefined,
+              costDetails: output.usage?.cost !== undefined ? { total: output.usage.cost } : undefined,
               metadata: output.metadata,
             });
             generation.end();