Isolate core tests from shell and rollout persistence (#33858)

## Why

The shared core test harness does not need to spawn a login shell for every
test, and approval scenarios exercise tool behavior rather than rollout
persistence.

## What changed

- Disable `ShellSnapshot` by default in `test_codex()`; snapshot-specific tests
  can continue to opt in explicitly.
- Give each approval scenario a named in-memory thread store so it does not use
  durable rollout storage.

GitOrigin-RevId: 787670857aa4f77d5fc442ec1e0bfe6d22314f63

codex-rs/core/tests/common/test_codex.rs

@@ -1260,6 +1260,11 @@ pub fn test_codex() -> TestCodexBuilder {
                 .features
                 .disable(Feature::Apps)
                 .expect("test config should allow Apps override");
+            // Snapshot tests opt in explicitly; avoid spawning login shells for every test.
+            config
+                .features
+                .disable(Feature::ShellSnapshot)
+                .expect("test config should allow ShellSnapshot override");
         })],
         auth: CodexAuth::from_api_key("dummy"),
         pre_build_hooks: vec![],

codex-rs/core/tests/suite/approvals.rs

@@ -5,6 +5,7 @@ use anyhow::Result;
 use codex_config::types::ApprovalsReviewer;
 use codex_core::CodexThread;
 use codex_core::config::Constrained;
+use codex_core::config::ThreadStoreConfig;
 use codex_core::sandboxing::SandboxPermissions;
 use codex_features::Feature;
 use codex_protocol::approvals::NetworkApprovalProtocol;
@@ -1846,8 +1847,13 @@ async fn run_scenario(scenario: &ScenarioSpec) -> Result<()> {
     let model_override = scenario.model_override;
     let model = model_override.unwrap_or("gpt-5.4");
     let policy_src = scenario.action.policy_src();
+    let thread_store_id = format!("approval-scenario-{}", scenario.name);
 
     let mut builder = test_codex().with_model(model).with_config(move |config| {
+        // These scenarios assert tool behavior, not rollout persistence.
+        config.experimental_thread_store = ThreadStoreConfig::InMemory {
+            id: thread_store_id,
+        };
         config.permissions.approval_policy = Constrained::allow_any(approval_policy);
         config
             .set_legacy_sandbox_policy(sandbox_policy.clone())