fix: resume lookup for gitignored CODEX_HOME (#5311)

Walk the sessions tree instead of using file_search so gitignored
CODEX_HOME directories can resume sessions. Add a regression test that
covers a .gitignore'd sessions directory.

Fixes #5247
Fixes #5412

---------

Co-authored-by: Owen Lin <owen@openai.com>

codex-rs/app-server/src/fuzzy_file_search.rs

@@ -46,6 +46,7 @@ pub(crate) async fn run_fuzzy_file_search(
                 threads,
                 cancel_flag,
                 COMPUTE_INDICES,
+                true,
             ) {
                 Ok(res) => Ok((root, res)),
                 Err(err) => Err((root, err)),