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/tools/code_mode/mod.rs

@@ -318,6 +318,7 @@ async fn call_nested_tool(
         tool_name,
         call_id: format!("{PUBLIC_TOOL_NAME}-{}", uuid::Uuid::new_v4()),
         payload,
+        encrypted_function_args: None,
     };
     let result = tool_runtime
         .handle_tool_call_with_source(