Handle partial answers consistently across agent workflows (#51249)

## Why

Partial answers contain answer fragments while a turn is still in progress. Treating them as final answers defers mailbox delivery and can count empty fragments as empty final responses; filtering them from forked histories and guardian transcripts loses answer content.

## What changed

- Allow mailbox preemption after `partial_answer` messages and keep mailbox delivery open.
- Preserve partial answers in forked agent histories and include them as protected assistant entries in guardian transcripts.
- Exclude empty partial answers from empty-final-response accounting for goals.

## Testing

Add regression coverage for partial-answer phases in live events, sampling continuations, follow-up requests, and forked child requests, plus mailbox preemption, goal accounting, and guardian transcript inclusion.

GitOrigin-RevId: 208c8933e211b42b6c2d0bec6952527b8c9c120e

codex-rs/core/src/agent/control/spawn.rs

@@ -89,7 +89,10 @@ fn keep_forked_rollout_item(item: &RolloutItem, preserve_context_baselines: bool
         RolloutItem::ResponseItem(envelope) => match &envelope.item {
             ResponseItem::Message { role, phase, .. } => match role.as_str() {
                 "system" | "developer" | "user" => true,
-                "assistant" => *phase == Some(MessagePhase::FinalAnswer),
+                "assistant" => matches!(
+                    phase,
+                    Some(MessagePhase::PartialAnswer | MessagePhase::FinalAnswer)
+                ),
                 _ => false,
             },
             ResponseItem::FunctionCallOutput { call_id: None, .. }

codex-rs/core/src/agent/control_tests.rs

@@ -2449,6 +2449,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                     ),
                 },
                 assistant_message("parent commentary", Some(MessagePhase::Commentary)),
+                assistant_message("parent answer fragment", Some(MessagePhase::PartialAnswer)),
                 assistant_message("parent final answer", Some(MessagePhase::FinalAnswer)),
                 standalone_output,
                 assistant_message("parent unknown phase", /*phase*/ None),
@@ -2529,6 +2530,22 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
     };
     assert!(assistant_position(&parent_thread.session.clone_history().await).is_some());
     assert_eq!(assistant_position(&history), None);
+    let expected_partial_answer = parent_thread
+        .session
+        .clone_history()
+        .await
+        .raw_items()
+        .find(|item| {
+            matches!(
+                item,
+                ResponseItem::Message {
+                    phase: Some(MessagePhase::PartialAnswer),
+                    ..
+                }
+            )
+        })
+        .cloned()
+        .expect("parent answer fragment should be recorded");
     let expected_final_answer = parent_thread
         .session
         .clone_history()
@@ -2587,6 +2604,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
     let expected_history = [
         expected_parent_seed,
         expected_developer_message,
+        expected_partial_answer,
         expected_final_answer,
         expected_standalone_output,
         ContextualUserFragment::into(MultiAgentRoleInstructions::Configured(