The xAI prompt_cache_key block carried two long comment paragraphs
that either restated setdefault semantics, narrated the SDK
type-validation mechanism, or recapped the historical motivation for
the extra_body indirection — all already covered by the test
docstring at test_xai_responses_sends_cache_key_via_extra_body
(which links to the xAI docs). Also restored the truncated link in
the body-injection comment.
No behavior change.
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