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/core/tests/common/Cargo.toml

@@ -10,8 +10,10 @@ path = "lib.rs"
 anyhow = { workspace = true }
 assert_cmd = { workspace = true }
 codex-core = { workspace = true }
+notify = { workspace = true }
 regex-lite = { workspace = true }
 serde_json = { workspace = true }
 tempfile = { workspace = true }
 tokio = { workspace = true, features = ["time"] }
+walkdir = { workspace = true }
 wiremock = { workspace = true }