Harden core test fixture startup assertions (#41194)

## Why

Test fixture setup can take more than five seconds on loaded CI workers, and
failures returned from setup can be obscured when downstream mocks verify their
expectations during teardown.

## What changed

- Add a shared `expect_startup` helper with a 30-second timeout that panics on
  either timeout or setup failure before mock teardown runs.
- Use the helper for remote-environment and unified-exec fixture setup, and
  allow the same startup window for the initial exec-server requests.

## Testing

Add paused-time tests covering successful startup after five seconds, timeout
reporting, and propagation of setup errors in the presence of unmet mocks.

GitOrigin-RevId: 39db69bde2983acd023a1911a6fad106674e169a

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

@@ -0,0 +1,43 @@
+//! Keeps startup failures visible even when downstream mocks have unmet expectations.
+
+use super::expect_startup;
+use crate::responses::ev_completed;
+use crate::responses::mount_sse_sequence;
+use crate::responses::sse;
+use crate::responses::start_mock_server;
+use anyhow::Result;
+use anyhow::anyhow;
+use pretty_assertions::assert_eq;
+use std::future::pending;
+use std::future::ready;
+use std::time::Duration;
+use tokio::time::sleep;
+
+#[tokio::test(start_paused = true)]
+async fn startup_can_complete_after_five_seconds() {
+    let value = expect_startup(async {
+        sleep(Duration::from_secs(/*secs*/ 6)).await;
+        Ok("ready")
+    })
+    .await;
+    assert_eq!(value, "ready");
+}
+
+#[tokio::test(start_paused = true)]
+#[should_panic(expected = "test fixture startup timed out")]
+async fn startup_timeout_is_not_masked_by_mock_expectations() {
+    let server = start_mock_server().await;
+    mount_sse_sequence(&server, vec![sse(vec![ev_completed("unused")])]).await;
+
+    expect_startup(pending::<Result<()>>()).await;
+}
+
+#[tokio::test(start_paused = true)]
+#[should_panic(expected = "injected startup failure")]
+async fn startup_error_is_not_masked_by_mock_expectations() {
+    let server = start_mock_server().await;
+    mount_sse_sequence(&server, vec![sse(vec![ev_completed("unused")])]).await;
+    let startup: Result<()> = Err(anyhow!("injected startup failure"));
+
+    expect_startup(ready(startup)).await;
+}