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, .. }