Centralize turn environment selection state (#38461)

## What changed

- Store `TurnEnvironmentSelection` directly on each resolved `TurnEnvironment`
  instead of copying its environment ID, working directory, and workspace roots
  into separate fields.
- Update environment resolution, tool execution, approvals, and related tests to
  access selection state through the retained value.

GitOrigin-RevId: ab723d21e112b8af5bcaf1f772a5e376553312bc

codex-rs/core/src/mcp_tool_call.rs

@@ -802,7 +802,7 @@ fn sandbox_cwd_for_mcp_server(step_context: &StepContext, environment_id: &str)
     if let Some(environment) = step_context
         .environments
         .turn_environments()
-        .find(|environment| environment.environment_id == environment_id)
+        .find(|environment| environment.selection.environment_id == environment_id)
     {
         return Some(environment.cwd().clone());
     }