Use step environments for Guardian approval reviews (#37618)

## Why

Deferred environments can become ready after a turn starts. Guardian approval
reviews must use the environment selected for the current step so their working
directory and permission context are not taken from the stale turn snapshot.

## What changed

- Carry `StepContext` through tool execution and approval handling.
- Add a Guardian review context that pairs the parent turn with the current
  step's environment snapshot.
- Use that snapshot for Guardian prompts, reviewer sessions, and delegated
  reviewer threads while retaining turn-level fallbacks for callers without a
  step context.

## Testing

Added an integration test that makes a deferred remote environment ready before
an escalated command and verifies Guardian receives that environment's working
directory and denied-read policy instead of the initial local environment's.

GitOrigin-RevId: 779096bc7d0e0c7453a009c02141029714b9b407

codex-rs/core/src/mcp_tool_call.rs

@@ -1363,7 +1363,7 @@ async fn maybe_request_mcp_tool_approval(
         let review_id = new_guardian_review_id();
         let decision = review_approval_request(
             sess,
-            turn_context,
+            step_context,
             review_id.clone(),
             build_guardian_mcp_tool_review_request(call_id, invocation, Some(metadata)),
             Default::default(),