test(codex): give the blocked-replacement handshake a workable deadline

[AlexanderKugel]

Fixes #124.

Problem

cancellation_while_replacement_startup_is_blocked_aborts_request_state waits for the replacement WebSocket handshake before it asserts anything. That wait had a two-second deadline:

tokio::time::timeout(std::time::Duration::from_secs(2), replacement_accepted_rx)
    .await
    .expect("replacement startup did not reach the blocked handshake")

The handshake takes about 2.5 seconds here, so the wait expired every time and the test failed before reaching its assertions:

panicked at src/providers/codex/mod.rs:2532:14:
replacement startup did not reach the blocked handshake: Elapsed(())

Change

Raise that one deadline to five seconds.

The assertion under test is cancellation cleanup, not handshake latency, so the deadline only needs enough headroom to stop deciding the result.

Verification

• The test passes 5 runs out of 5, each completing in about 2.6 seconds.
• cargo test --lib: 861 passed, 0 failed.
• cargo fmt --check clean.

Tested on Linux 6.18 (WSL2), rustc 1.97.1. The margin is now roughly double the observed handshake time, so a slower machine has room as well.