Fix input/output getting misrouted by mixed usage-schema keys

The previous cached-tokens commit added cache_read_input_tokens (a
flat-schema key) alongside prompt_tokens/completion_tokens (OpenAI's
native schema) in the same usageDetails object. Langfuse's ingestion
explicitly does not support mixing those two schemas, and doing so
broke generation.update()'s output field - confirmed live: prompt/
response content was landing in the Usage panel instead of Input/
Output.

Fixed by staying entirely within OpenAI's native schema: cached
tokens now nest under prompt_tokens_details.cached_tokens instead of
using the flat cache_read_input_tokens key. Cast past the installed
@langfuse/tracing SDK's usageDetails type (which only exposes a
3-field camelCase shape with no _details nesting, unlike the wire
schema in @langfuse/core) since the nesting is valid on the wire even
though the SDK's public type doesn't model it.

Verified: full suite (82/82) passing, tsc/eslint clean.

test/core/langfuse/LangfuseTracer.test.ts

@@ -78,7 +78,7 @@ describe('LangfuseTracer', () => {
     expect(generationEnd).toHaveBeenCalledTimes(1);
   });
 
-  it('maps cachedTokens to the flat cache_read_input_tokens key', () => {
+  it('maps cachedTokens to the OpenAI-native nested prompt_tokens_details.cached_tokens, not a mixed flat key', () => {
     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: 'gpt-4o-mini', input: ['hi'] });
@@ -93,7 +93,7 @@ describe('LangfuseTracer', () => {
           prompt_tokens: 100,
           completion_tokens: 20,
           total_tokens: 120,
-          cache_read_input_tokens: 80,
+          prompt_tokens_details: { cached_tokens: 80 },
         },
       })
     );