Treat code-mode collaboration messages as plaintext (#55633)

## What changed

Classify messages from code-mode collaboration calls as `AgentMessage::Plaintext` so they receive the context wrapper for plaintext messages. Apply this to `spawn_agent`, `send_message`, and `followup_task`.

## Testing

Add a regression test that spawns an agent through `exec` and verifies that the child receives the task as plaintext with sender and recipient attribution.

GitOrigin-RevId: 495c70ea13fd2bb06989e5aacab60beed0c19a56

codex-rs/core/src/tools/handlers/multi_agents_v2.rs

@@ -58,6 +58,7 @@ fn agent_message_from_tool(
     if matches!(
         source,
         crate::tools::context::ToolCallSource::DirectPlaintextMessage
+            | crate::tools::context::ToolCallSource::CodeMode { .. }
     ) {
         AgentMessage::Plaintext(message)
     } else {