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/session/mcp.rs

@@ -392,7 +392,7 @@ impl Session {
                     sandbox.windows_sandbox_private_desktop =
                         config.permissions.windows_sandbox_private_desktop;
                     sandbox.use_legacy_landlock = config.features.use_legacy_landlock();
-                    (environment.environment_id.clone(), sandbox)
+                    (environment.selection.environment_id.clone(), sandbox)
                 })
                 .collect::<HashMap<_, _>>()
         } else {