analytics: add responses api call schema

[rhan-oai]

Why

We need a Responses API call analytics event that can support context-window-health analysis without logging raw prompt, tool, or model content. This PR adds the analytics-side event schema and item-metadata extraction primitives that describe the shape and approximate size of the ResponseItems involved in a Responses API call.

What changed

This adds the codex_responses_api_call_event request shape, including thread and turn identifiers, thread lifecycle metadata, client/runtime metadata, Responses API identifiers, model metadata, terminal status, timing, token counts, and per-item metadata.

It introduces CodexResponsesApiCallInput, CodexResponsesApiCallFact, CodexResponsesApiCallStatus, CodexResponsesApiItemMetadata, CodexResponsesApiItemPhase, and CodexResponseItemType in codex-rs/analytics/src/facts.rs. The public API intentionally exposes only the core-facing input/status types from codex_analytics; the event fact and item metadata remain analytics-internal.

It also adds codex-rs/analytics/src/response_items.rs, which maps protocol ResponseItems into metadata-only analytics records. The mapper captures fields such as item phase, item index, response item type, role, status, message phase, call id, tool name, payload byte size, text part count, and image part count. It avoids raw content while preserving enough structure to analyze context-window composition.

Finally, this adds small serialization/byte helpers in codex-rs/analytics/src/lib.rs that are shared by the response item metadata mapper.

Verification

Verified with cargo test -p codex-analytics, which covers the new response item metadata mapping tests.

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• core: emit responses api call analytics #18030
• analytics: ingest responses api call events #18029
• -> analytics: add responses api call schema #18028

[rhan-oai]

to pass lint tests, removed in next commit