Fix prompt_tokens_details always null when num_cached_tokens is 0

[factnn]

Summary

Fixes #44961: usage.prompt_tokens_details was always null in API responses despite --enable-prompt-tokens-details, because num_cached_tokens=0 (no cache hits) is falsy in Python.

PR #18149 wired num_cached_tokens through the engine but the serving layer condition if num_cached_tokens treated 0 the same as None, so prompt_tokens_details was never populated for any request.

Fix

Change and num_cached_tokens → and num_cached_tokens is not None in three locations:

File	Path
chat_completion/serving.py	streaming usage chunk (L884)
chat_completion/serving.py	non-streaming final usage (L1308)
completion/serving.py	streaming usage chunk (L446)

Test Plan

Logic verification:

num_cached_tokens | old behavior        | new behavior
None              | null (correct)      | null (correct)
0                 | null (WRONG)        | {"cached_tokens": 0}
5                 | {"cached_tokens":5} | {"cached_tokens":5}

AI Assistance

This PR was developed with AI assistance (Claude). All changed lines have been reviewed by the human submitter.

Co-authored-by: Claude noreply@anthropic.com

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @factnn.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[factnn]

This fix has already been applied on main — closing as resolved upstream.