Support plaintext collaboration tool messages (#35845)

## What changed

- Preserve `encrypted_function_args` on function calls so an empty list can mark plaintext collaboration arguments across request replay.
- Deliver `spawn_agent`, `send_message`, and `followup_task` payloads as structured plaintext agent messages when that marker is present; retain encrypted delivery otherwise.
- Redact plaintext collaboration arguments from tool and communication logs, and omit the metadata when sending requests to non-OpenAI providers.

## Testing

- Cover serialization of empty encrypted-argument metadata and plaintext versus encrypted subagent message delivery.
- Verify plaintext tool arguments are redacted and provider-specific metadata is removed from non-OpenAI requests.

GitOrigin-RevId: 64db98ff0b61a3af2f04ed609292363f2e2362a8

codex-rs/core/src/agent_communication.rs

@@ -56,11 +56,10 @@ pub(crate) fn emit_agent_communication_send(
             state = "send",
             sender_thread_id = %context.sender_thread_id,
             receiver_thread_id = %receiver_thread_id,
-            content = if communication.content.is_empty() {
-                communication.encrypted_content.as_deref().unwrap_or_default()
-            } else {
-                communication.content.as_str()
-            },
+            content = communication
+                .encrypted_content
+                .as_deref()
+                .unwrap_or("[plaintext]"),
         },
         "agent communication"
     );