- Added `n_cache_read_input_tokens` field to `server_task_result_cmpl_final` and `server_task_result_cmpl_partial` structs
- Populated `cache_read_input_tokens` in JSON output for both final and streaming responses
- Ensured `cache_read_input_tokens` is non-negative by clamping to zero if negative
- Updated unit tests to validate presence, type, and non-negativity of `cache_read_input_tokens` in usage metrics
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