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/tui/src/file_search.rs

@@ -172,6 +172,7 @@ impl FileSearchManager {
                 NUM_FILE_SEARCH_THREADS,
                 cancellation_token.clone(),
                 compute_indices,
+                true,
             )
             .map(|res| res.matches)
             .unwrap_or_default();