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/agents_md.rs

@@ -65,7 +65,7 @@ pub(crate) async fn load_project_instructions(
         match read_agents_md(
             config,
             filesystem.as_ref(),
-            &turn_environment.environment_id,
+            &turn_environment.selection.environment_id,
             turn_environment.cwd(),
             remaining,
         )
@@ -80,7 +80,7 @@ pub(crate) async fn load_project_instructions(
             Ok(None) => {}
             Err(e) => {
                 error!(
-                    environment_id = turn_environment.environment_id,
+                    environment_id = turn_environment.selection.environment_id,
                     "error trying to find AGENTS.md docs: {e:#}"
                 );
             }