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/thread_manager_tests.rs

@@ -203,6 +203,7 @@ fn truncates_before_requested_user_message() {
             name: "tool".to_string(),
             namespace: None,
             arguments: "{}".to_string(),
+            encrypted_function_args: None,
             internal_chat_message_metadata_passthrough: None,
         },
         assistant_msg("a4"),