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/tools/runtimes/shell/unix_escalation.rs

@@ -176,7 +176,7 @@ pub(super) async fn try_run_zsh_fork(
         ctx.session
             .services
             .exec_policy
-            .current_for_prefix_rules(ctx.turn.allow_prefix_rules())
+            .current_for_prefix_rules(ctx.step_context.turn.allow_prefix_rules())
             .as_ref()
             .clone(),
     ));
@@ -289,7 +289,7 @@ pub(crate) async fn prepare_unified_exec_zsh_fork(
         ctx.session
             .services
             .exec_policy
-            .current_for_prefix_rules(ctx.turn.allow_prefix_rules())
+            .current_for_prefix_rules(ctx.step_context.turn.allow_prefix_rules())
             .as_ref()
             .clone(),
     ));

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 {