Use the step context for command approval prefix rules (#37641)

## What changed

Read `allow_prefix_rules` from the turn attached to the active step context
when selecting the exec policy and constructing unified exec approval requests.

GitOrigin-RevId: d2d26e88b9cefa28bb2df5b9f71aa7b30b7ea331

codex-rs/core/src/unified_exec/process_manager.rs

@@ -1174,7 +1174,7 @@ impl UnifiedExecProcessManager {
                     request.sandbox_permissions
                 },
                 prefix_rule: request.prefix_rule.clone(),
-                allow_prefix_rules: context.turn.allow_prefix_rules(),
+                allow_prefix_rules: context.step_context.turn.allow_prefix_rules(),
             })
             .await;
         let req = UnifiedExecToolRequest {