Normalize context compaction before API requests - #27289

Summary

Fixes #27269.

Remote compaction can yield ResponseItem::ContextCompaction, but the Responses API rejects context_compaction in request input. This change centralizes model/API-bound normalization so encrypted ContextCompaction payloads are sent as the API-supported compaction item, and empty context-compaction markers are omitted from model history.

The app-server/UI lifecycle path remains unchanged: UI compaction markers still flow through TurnItem::ContextCompaction and v2 ThreadItem::ContextCompaction.

Root Cause

Remote compaction post-processing preserved ResponseItem::ContextCompaction in replacement history. A later model request could replay that history into /responses, producing the API wire enum error:

Invalid value: context_compaction

What Changed

• Added one helper, model_api_safe_response_item, for API-safe ResponseItem normalization.
• Reused that helper from Prompt::get_formatted_input() and remote compaction replacement-history processing.
• Converted encrypted context_compaction items to compaction before model/API use.
• Dropped empty context_compaction items from model/API-bound history because they are UI lifecycle markers; the UI path uses TurnItem::ContextCompaction.
• Added regression coverage for Rust enum normalization and serialized wire shape.

Validation

• just fmt
• just test -p codex-core formatted_input_normalizes_context_compaction_for_responses_api process_compacted_history_normalizes_context_compaction_items
  • 2 tests passed
  • recipe bench-smoke tail completed
• Broad just test -p codex-core was attempted. The new compaction tests passed, but the full run failed in unrelated sandbox/environment categories:
  • PATH alias creation denied in sandbox
  • missing target/debug/test_stdio_server for MCP stdio tests
  • sandbox-exec: sandbox_apply: Operation not permitted
  • timing/interrupt/unified exec tests sensitive to this managed sandbox
• just fix -p codex-core

Adversarial Review

• Responses API wire enum: Prompt::get_formatted_input() now normalizes API-bound input, and the regression test asserts serialized JSON contains "type":"compaction" and not context_compaction.
• UI behavior: TurnItem::ContextCompaction remains unchanged, so app-server v2 clients still receive the canonical UI compaction item.
• Empty marker rationale: ContextCompaction { encrypted_content: None } carries no model-visible encrypted compaction payload. It is a UI lifecycle marker, so omitting it from model/API history avoids sending an unsupported enum without losing model history.
• Resume/replay/version skew: persisted or replayed context_compaction response items are normalized at the model request boundary, and new remote replacement histories are cleaned before persistence.
• Future item kinds: the normalization helper enumerates all current passthrough ResponseItem variants instead of using a wildcard arm, so new variants require an explicit review decision.