Record saved multi-agent opt-outs in session telemetry (#59478)

## Why

Saved `features.multi_agent = false` settings can be overridden by a higher-precedence session setting. Session telemetry should retain visibility into those saved opt-outs.

## What changed

Add `config.legacy_multi_agent_disabled_sources` to `codex.conversation_starts`, reporting opt-outs from user configuration, project configuration, and managed requirements. Emit comma-separated source labels, or `none` when no opt-outs are found.

## Testing

Add a regression test confirming that a user opt-out is still reported when a CLI override enables multi-agent support for the session.

GitOrigin-RevId: 488bd317d67e9ee87c5903ae51f466e0a63c05eb

codex-rs/core/src/session/session.rs

@@ -1512,6 +1512,7 @@ impl Session {
                     .permissions
                     .legacy_sandbox_policy(session_configuration.cwd().as_path()),
                 mcp_server_names.iter().map(String::as_str).collect(),
+                &crate::config::legacy_multi_agent_disabled_sources(&config.config_layer_stack),
             );
 
             let default_shell = session_configuration