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/guardian-context/src/transcript_tests.rs

@@ -131,6 +131,15 @@ fn registered_transcript_filters_roles_and_preserves_node_repl_tool_attribution(
             phase: None,
             internal_chat_message_metadata_passthrough: None,
         },
+        ResponseItem::Message {
+            id: None,
+            role: "assistant".to_string(),
+            content: vec![ContentItem::OutputText {
+                text: "The first result is ready.".to_string(),
+            }],
+            phase: Some(MessagePhase::PartialAnswer),
+            internal_chat_message_metadata_passthrough: None,
+        },
         ResponseItem::Message {
             id: None,
             role: "assistant".to_string(),
@@ -187,6 +196,10 @@ fn registered_transcript_filters_roles_and_preserves_node_repl_tool_attribution(
                     "Inspect the workspace."
                 ),
                 entry(ConversationTranscriptEntryKind::Developer, &approved_action),
+                entry(
+                    ConversationTranscriptEntryKind::ProtectedAssistant,
+                    "The first result is ready."
+                ),
                 entry(
                     ConversationTranscriptEntryKind::ProtectedAssistant,
                     "Inspection complete."