Record sandbox backends in command execution analytics (#47568)

## Why

Command execution analytics lack the observed sandbox backend. Older execution servers may omit this information, so an unknown backend must remain distinguishable from an explicitly unsandboxed process.

## What changed

- Carry optional `sandbox_type` metadata through command execution items and emit its metric tag as `sandbox_backend` in analytics.
- Propagate the observed backend through unified execution completion and failure events, including asynchronous exits.
- Preserve missing backend information as `None` and mark executed user shell commands as `Some(SandboxType::None)`.
- Exclude the metadata from serialized command items and generated schemas and TypeScript types.

## Testing

Extend tests for missing and explicit executor backends, backend propagation on failed execution, and analytics payloads, including platform-specific backend tags in app-server integration coverage.

GitOrigin-RevId: 178fe46824a6194910802ec02168ed40ee0b2019

codex-rs/protocol/src/items.rs

@@ -242,6 +242,11 @@ pub struct ModelInvocationContext {
 
 #[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema, PartialEq)]
 pub struct CommandExecutionItem {
+    /// Observed process backend for live analytics; None means unknown or not launched.
+    #[serde(skip)]
+    #[schemars(skip)]
+    #[ts(skip)]
+    pub sandbox_type: Option<crate::sandbox::SandboxType>,
     #[serde(skip)]
     #[schemars(skip)]
     #[ts(skip)]

codex-rs/protocol/src/protocol.rs

@@ -5581,6 +5581,7 @@ mod tests {
             started_at_ms: 10,
             item: TurnItem::CommandExecution(CommandExecutionItem {
                 model_context: None,
+                sandbox_type: None,
                 id: "exec-1".into(),
                 plugin_id: Some("sample@openai-curated".into()),
                 script_path: Some("scripts/run.py".into()),
@@ -5608,6 +5609,7 @@ mod tests {
             completed_at_ms: 20,
             item: TurnItem::CommandExecution(CommandExecutionItem {
                 model_context: None,
+                sandbox_type: None,
                 id: "exec-1".into(),
                 plugin_id: Some("sample@openai-curated".into()),
                 script_path: Some("scripts/run.py".into()),