Fix compaction context reinjection and model baselines (#12252)

## Summary
- move regular-turn context diff/full-context persistence into
`run_turn` so pre-turn compaction runs before incoming context updates
are recorded
- after successful pre-turn compaction, rely on a cleared
`reference_context_item` to trigger full context reinjection on the
follow-up regular turn (manual `/compact` keeps replacement history
summary-only and also clears the baseline)
- preserve `<model_switch>` when full context is reinjected, and inject
it *before* the rest of the full-context items
- scope `reference_context_item` and `previous_model` to regular user
turns only so standalone tasks (`/compact`, shell, review, undo) cannot
suppress future reinjection or `<model_switch>` behavior
- make context-diff persistence + `reference_context_item` updates
explicit in the regular-turn path, with clearer docs/comments around the
invariant
- stop persisting local `/compact` `RolloutItem::TurnContext` snapshots
(only regular turns persist `TurnContextItem` now)
- simplify resume/fork previous-model/reference-baseline hydration by
looking up the last surviving turn context from rollout lifecycle
events, including rollback and compaction-crossing handling
- remove the legacy fallback that guessed from bare `TurnContext`
rollouts without lifecycle events
- update compaction/remote-compaction/model-visible snapshots and
compact test assertions (including remote compaction mock response
shape)

## Why
We were persisting incoming context items before spawning the regular
turn task, which let pre-turn compaction requests accidentally include
incoming context diffs without the new user message. Fixing that exposed
follow-on baseline issues around `/compact`, resume/fork, and standalone
tasks that could cause duplicate context injection or suppress
`<model_switch>` instructions.

This PR re-centers the invariants around regular turns:
- regular turns persist model-visible context diffs/full reinjection and
update the `reference_context_item`
- standalone tasks do not advance those regular-turn baselines
- compaction clears the baseline when replacement history may have
stripped the referenced context diffs

## Follow-ups (TODOs left in code)
- `TODO(ccunningham)`: fix rollback/backtracking baseline handling more
comprehensively
- `TODO(ccunningham)`: include pending incoming context items in
pre-turn compaction threshold estimation
- `TODO(ccunningham)`: inject updated personality spec alongside
`<model_switch>` so some model-switch paths can avoid forced full
reinjection
- `TODO(ccunningham)`: review task turn lifecycle
(`TurnStarted`/`TurnComplete`) behavior and emit task-start context
diffs for task types that should have them (excluding `/compact`)

## Validation
- `just fmt`
- CI should cover the updated compaction/resume/model-visible snapshot
expectations and rollout-hydration behavior
- I did **not** rerun the full local test suite after the latest
resume-lookup / rollout-persistence simplifications

codex-rs/core/tests/suite/resume_warning.rs

@@ -6,9 +6,13 @@ use codex_core::protocol::EventMsg;
 use codex_core::protocol::InitialHistory;
 use codex_core::protocol::ResumedHistory;
 use codex_core::protocol::RolloutItem;
+use codex_core::protocol::TurnCompleteEvent;
 use codex_core::protocol::TurnContextItem;
+use codex_core::protocol::TurnStartedEvent;
+use codex_core::protocol::UserMessageEvent;
 use codex_core::protocol::WarningEvent;
 use codex_protocol::ThreadId;
+use codex_protocol::config_types::ModeKind;
 use core::time::Duration;
 use core_test_support::load_default_config_for_test;
 use core_test_support::wait_for_event;
@@ -19,8 +23,9 @@ fn resume_history(
     previous_model: &str,
     rollout_path: &std::path::Path,
 ) -> InitialHistory {
+    let turn_id = "resume-warning-seed-turn".to_string();
     let turn_ctx = TurnContextItem {
-        turn_id: None,
+        turn_id: Some(turn_id.clone()),
         cwd: config.cwd.clone(),
         approval_policy: config.permissions.approval_policy.value(),
         sandbox_policy: config.permissions.sandbox_policy.get().clone(),
@@ -38,7 +43,24 @@ fn resume_history(
 
     InitialHistory::Resumed(ResumedHistory {
         conversation_id: ThreadId::default(),
-        history: vec![RolloutItem::TurnContext(turn_ctx)],
+        history: vec![
+            RolloutItem::EventMsg(EventMsg::TurnStarted(TurnStartedEvent {
+                turn_id: turn_id.clone(),
+                model_context_window: None,
+                collaboration_mode_kind: ModeKind::Default,
+            })),
+            RolloutItem::EventMsg(EventMsg::UserMessage(UserMessageEvent {
+                message: "seed".to_string(),
+                images: None,
+                local_images: vec![],
+                text_elements: vec![],
+            })),
+            RolloutItem::TurnContext(turn_ctx),
+            RolloutItem::EventMsg(EventMsg::TurnComplete(TurnCompleteEvent {
+                turn_id,
+                last_agent_message: None,
+            })),
+        ],
         rollout_path: rollout_path.to_path_buf(),
     })
 }