Update tests for default-enabled update_plan (#41630)

## What changed

- Cover the default, explicitly enabled, and explicitly disabled states of
  `tools.update_plan.enabled`.
- Verify prompt tool lists remain consistent across requests, including when
  custom base and developer instructions are configured.
- Explicitly enable `update_plan` in existing fixtures that depend on the tool.

GitOrigin-RevId: 3be279cb5195dfc5892c2c8ff6a1461fac879309

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

@@ -3594,6 +3594,7 @@ async fn fork_startup_context_then_first_turn_diff_snapshot() -> anyhow::Result<
     .await;
 
     let mut builder = test_codex().with_config(|config| {
+        config.update_plan_enabled = true;
         config.permissions.approval_policy =
             codex_config::Constrained::allow_any(AskForApproval::OnRequest);
     });