Use step environments for extension turn input (#37336)

## Why

Environment readiness can change between sampling requests. Extension turn-input contributors should receive the same refreshed environment snapshot used by the current step.

## What changed

Build `TurnInputContext.environments` from `StepContext` so newly ready environments are included in extension input for the current request.

GitOrigin-RevId: ee4444072162317974cd2bbfd57ffcf9a9445391

codex-rs/core/src/session/turn.rs

@@ -880,7 +880,7 @@ async fn build_extension_turn_input_items(
         return Some(Vec::new());
     }
 
-    let environments = turn_context
+    let environments = step_context
         .environments
         .turn_environments()
         .enumerate()