Attribute command and plugin analytics to the invoking model (#45445)

## Why

Command execution and plugin measurement events lack model and reasoning-effort labels. Attribution needs to reflect the step that invoked the command, even when model settings change before a background process finishes.

## What changed

- Add `model_slug` and `reasoning_effort` to command execution and plugin measurement analytics.
- Capture model context from resolved step settings and carry it through execution, approval, Guardian review, and plugin metrics collection.
- Preserve the first command-start model context when subsequent start notifications arrive.
- Keep the carried context out of serialized protocol items and generated schemas.

## Testing

Extend analytics tests to cover model switches before invocation and during background execution, default reasoning effort, Guardian-denied commands, and repeated start notifications retaining the original model context.

GitOrigin-RevId: af90e1c0d39bab625f2e89786085b61a9b96c0ce

codex-rs/protocol/src/items.rs

@@ -232,8 +232,19 @@ pub fn is_safe_plugin_relative_path(path: &str) -> bool {
         })
 }
 
+/// Immutable model labels carried within command lifecycle events for analytics.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ModelInvocationContext {
+    pub model_slug: String,
+    pub reasoning_effort: Option<String>,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema, PartialEq)]
 pub struct CommandExecutionItem {
+    #[serde(skip)]
+    #[schemars(skip)]
+    #[ts(skip)]
+    pub model_context: Option<ModelInvocationContext>,
     pub id: String,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(optional)]