* fix: handle None values in _extract_usage to prevent TypeError
Some LLM providers (especially API proxies) may return None for
prompt_tokens and completion_tokens in the usage response. This
causes a TypeError when attempting arithmetic operations.
Added null checks with fallback to 0 for both prompt_tokens and
completion_tokens before performing calculations.
* refactor: use explicit None check and reuse cached variable
- Use `is None` instead of `or 0` to avoid masking unexpected falsy values
- Reuse `cached` variable for `input_cached` to avoid redundant calculation
* ruff format
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Co-authored-by: Soulter <905617992@qq.com>
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