Drop descendant channel posts from retained remote compaction v2 history (#47249)

## Why

Remote compaction v2 excluded descendant progress messages from retained history but kept `CHANNEL_POST` messages from those same agents.

## What changed

Treat descendant `CHANNEL_POST` messages like `MESSAGE` progress updates when selecting history to retain after remote compaction v2.

## Testing

Extend the remote compaction test to verify that a child channel post is included in the compaction input and omitted from the follow-up request.

GitOrigin-RevId: 204bf44b8cddf1e6dc1e48234f51d4ac2aaf2e9f

codex-rs/core/src/compact_remote_v2.rs

@@ -572,6 +572,7 @@ fn is_retained_for_remote_compaction_v2(
                 content.first(),
                 Some(AgentMessageInputContent::InputText { text })
                     if text.starts_with("Message Type: MESSAGE\n")
+                        || text.starts_with("Message Type: CHANNEL_POST\n")
             );
         let is_completion = matches!(
             content.first(),

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

@@ -912,6 +912,19 @@ async fn remote_compact_v2_reuses_compaction_trigger_for_followups() -> Result<(
             start_options: Default::default(),
         })
         .await?;
+    let channel_progress = "Message Type: CHANNEL_POST\nSender: /root/child\nChannel: progress\nMessage ID: aaaabbbbcccc\nThread ID: aaaabbbbcccc\nPayload:\nchild channel progress";
+    codex
+        .submit(Op::InterAgentCommunication {
+            communication: InterAgentCommunication::new(
+                AgentPath::root().join("child").expect("valid child path"),
+                AgentPath::root(),
+                Vec::new(),
+                channel_progress.to_string(),
+                /*trigger_turn*/ false,
+            ),
+            start_options: Default::default(),
+        })
+        .await?;
     codex
         .submit(Op::InterAgentCommunication {
             communication: InterAgentCommunication::new(
@@ -1012,6 +1025,12 @@ async fn remote_compact_v2_reuses_compaction_trigger_for_followups() -> Result<(
             .any(|item| item.to_string().contains("child progress")),
         "expected v2 compaction input to include the child progress update"
     );
+    assert!(
+        compact_request
+            .inputs_of_type("agent_message")
+            .iter()
+            .any(|item| item["content"][0]["text"].as_str() == Some(channel_progress))
+    );
     assert!(
         compact_request
             .inputs_of_type("agent_message")
@@ -1106,6 +1125,12 @@ async fn remote_compact_v2_reuses_compaction_trigger_for_followups() -> Result<(
             .all(|item| !item.to_string().contains("child progress")),
         "expected v2 follow-up request to omit the child progress update"
     );
+    assert!(
+        follow_up_request
+            .inputs_of_type("agent_message")
+            .iter()
+            .all(|item| item["content"][0]["text"].as_str() != Some(channel_progress))
+    );
     assert!(
         follow_up_request
             .inputs_of_type("agent_message")