core: share responses request builder with compact requests

[jif-oai]

Why

ModelClientSession and compact_conversation_history() were still rebuilding the same ResponsesApiRequest fields separately. That duplication makes it easy for normal /responses turns and compact requests to drift when request-shape changes land later, which is exactly the kind of cache-affecting divergence we want to avoid.

This follow-up keeps the scope small by extracting the shared request-construction logic into one helper and using it from both paths.

What changed

• move ResponsesApiRequest construction into a shared ModelClient::build_responses_request(...) helper in core/src/client.rs
• update the normal /responses streaming path to call that helper instead of the old ModelClientSession-local implementation
• update compact_conversation_history() to derive its compact payload from the same helper so model, instructions, input, tools, parallel_tool_calls, reasoning, and text stay aligned with normal request building
• add a unit test covering the shared helper's prompt cache key, installation metadata, and service_tier behavior

Verification

• cargo test -p codex-core build_responses_request_sets_shared_cache_and_metadata_fields
• cargo test -p codex-core --test all remote_compact_v2_reuses_context_compaction_for_followups

Docs

No docs update needed.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

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