Groq prompt cache hits now report cached token counts

Patch Changes

• cee7e45: fix (provider/groq): surface prompt cache reads in usage

  convertGroqUsage accepted prompt_tokens_details.cached_tokens but never read it, so cache hits were reported as cacheRead: undefined and the entire prompt was counted as noCache. Groq's implicit prompt caching now surfaces as usage.cachedInputTokens (mapped to cacheRead, subtracted from noCache). Groq has no cache-creation charge, so cacheWrite remains undefined.