Retain TUI delegation prompts in thread model history (#61210)

## What changed

Set `retain: true` on tool outputs submitted when TUI dynamic tools start a turn, requesting retention of delegation prompts in the thread's model history.

## Testing

Update session lifecycle assertions to require `retain: true` for both `create_thread` and `send_message_to_thread` requests.

GitOrigin-RevId: 095d0f6f687a67fbf0b5ffd98539f764d32e2acc

codex-rs/tui/src/app/tests/session_lifecycle_requests.rs

@@ -3276,6 +3276,7 @@ async fn check_dynamic_tool_requests(
         serde_json::json!({
             "name": "create_thread",
             "namespace": "codex_tui",
+            "retain": true,
             "output": format!(
                 "<codex_delegation>\n  <source_thread_id>{creation_source}</source_thread_id>\n  <input>Check &lt;main&gt; &amp; report</input>\n</codex_delegation>"
             )
@@ -3361,6 +3362,7 @@ async fn check_dynamic_tool_requests(
         serde_json::json!({
             "name": "send_message_to_thread",
             "namespace": "codex_tui",
+            "retain": true,
             "output": format!(
                 "<codex_delegation>\n  <source_thread_id>{thread_id}</source_thread_id>\n  <input>Follow &lt;up&gt; &amp; report</input>\n</codex_delegation>"
             )

codex-rs/tui/src/dynamic_tools.rs

@@ -1271,7 +1271,7 @@ async fn start_turn(
             // Older app-server/TUI versions are intentionally unsupported:
             // preserving tool authority takes precedence over legacy fallback.
             tool_output: Some(Box::new(TurnToolOutput {
-                retain: false,
+                retain: true,
                 name: tool.to_string(),
                 namespace: Some(NAMESPACE.to_string()),
                 output: FunctionCallOutputBody::Text(prompt),