test: reduce time dependency on test harness (#5053)

Tightened the CLI integration tests to stop relying on wall-clock
sleeps—new fs watcher helper waits for session files instead of timing
out, and SSE mocks/fixtures make the flows deterministic.

codex-rs/Cargo.toml

@@ -122,6 +122,7 @@ log = "0.4"
 maplit = "1.0.2"
 mime_guess = "2.0.5"
 multimap = "0.10.0"
+notify = "8.2.0"
 nucleo-matcher = "0.3.1"
 openssl-sys = "*"
 opentelemetry = "0.30.0"