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/core/src/session/step_context.rs

@@ -12,6 +12,7 @@ use codex_exec_server::ExecutorCapabilityDiscoverySnapshot;
 use codex_exec_server::ResolvedSelectedCapabilityRoot;
 use codex_mcp::McpBinding;
 use codex_otel::SessionTelemetry;
+use codex_protocol::items::ModelInvocationContext;
 use codex_protocol::protocol::TurnContextItem;
 
 /// Request-scoped state that may change between model sampling requests.
@@ -43,4 +44,14 @@ impl StepContext {
         item.summary = self.settings.reasoning_summary;
         item
     }
+
+    pub(crate) fn model_context(&self) -> ModelInvocationContext {
+        ModelInvocationContext {
+            model_slug: self.settings.model_info.slug.clone(),
+            reasoning_effort: self
+                .settings
+                .effective_reasoning_effort()
+                .map(|effort| effort.to_string()),
+        }
+    }
 }