Kimi K2 models use automatic prefix caching and return cache stats in
a nested field: usage.prompt_tokens_details.cached_tokens
This fixes issue #7073 where cacheRead was showing 0 for K2.5 users.
Also adds cached_tokens (top-level) for moonshot-v1 explicit caching API.
Closes#7073
* initial commit
* feat: implement deriveSessionTotalTokens function and update usage tests
* Added deriveSessionTotalTokens function to calculate total tokens based on usage and context tokens.
* Updated usage tests to include cases for derived session total tokens.
* Refactored session usage calculations in multiple files to utilize the new function for improved accuracy.
* fix: restore overflow truncation fallback + changelog/test hardening (#11551) (thanks @tyler6204)
