Classify additional generated context fragments (#40275)

## What changed

- Represent compaction summaries and Guardian-approved actions as typed contextual fragments.
- Inject subagent notifications through the same fragment path.
- Annotate the resulting request items with `compaction.summary`, `guardian.approved_action`, and `multi_agent.subagent_notification` content kinds while preserving their existing roles and text.

## Testing

- Verify the content-kind metadata on compacted summaries, approved actions, and subagent notifications.

GitOrigin-RevId: 01c93e7f08b2dc1c1e85ba69e82bfb5f56e6cca5

codex-rs/core/src/compact.rs

@@ -4,6 +4,8 @@ use std::time::Instant;
 use crate::Prompt;
 use crate::client::ModelClientSession;
 use crate::client_common::ResponseEvent;
+use crate::context::CompactionSummary;
+use crate::context::ContextualUserFragment;
 use crate::context::world_state::WorldState;
 use crate::hook_runtime::PostCompactHookOutcome;
 use crate::hook_runtime::PreCompactHookOutcome;
@@ -723,13 +725,9 @@ fn build_compacted_history_with_limit(
         summary_text.to_string()
     };
 
-    history.push(ResponseItemEnvelope::new(ResponseItem::Message {
-        id: None,
-        role: "user".to_string(),
-        content: vec![ContentItem::InputText { text: summary_text }],
-        phase: None,
-        internal_chat_message_metadata_passthrough: None,
-    }));
+    history.push(ResponseItemEnvelope::new(ContextualUserFragment::into(
+        CompactionSummary::new(summary_text),
+    )));
 
     history
 }