Use stable Guardian prompt cache keys

[jif-oai]

Why

Guardian review sessions are reusable across forks when their GuardianReviewSessionReuseKey is unchanged, but the underlying Responses request was still using the child thread ID as prompt_cache_key. That meant forked Guardian reviews that should share cache context produced different cache keys, reducing prompt cache reuse and weakening the reuse invariant.

What Changed

• Adds a ModelClient prompt cache key override and uses it for ResponsesApiRequest.prompt_cache_key.
• Computes Guardian review cache keys as guardian:<sha1(parent_thread_id:reuse_key)>, scoped to the parent thread plus the reuse-sensitive Guardian config.
• Wires session construction to apply that override only for Guardian sub-agent sessions.

Testing

• Added coverage that Guardian cache keys are stable for the same parent/reuse key, change when either the parent thread or reuse key changes, fit within the Responses API length limit, and are absent for non-Guardian sessions.
• Extended the parallel review test to assert forked Guardian reviews send the same prompt_cache_key.

[jif-oai]

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[jif-oai]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: cf06538e4c

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

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