Follow symlinks during file search (openai#4453)

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Closes openai#4452

This fixes a usability issue where users with symlinked folders in their
working directory couldn't search those files using the `@` file search
feature.

## Rationale

The "bug" was in the file search implementation in
`codex-rs/file-search/src/lib.rs`. The `WalkBuilder` was using default
settings which don't follow symlinks, causing two related issues:

1. Partial search results: The `@` search would find symlinked
directories but couldn't find files inside them
2. Inconsistent behavior: Users expect symlinked folders to behave like
regular folders in search results.

## Root cause

The `ignore` crate's `WalkBuilder` defaults to `.follow_links(false)`
[[source](https://github.com/BurntSushi/ripgrep/blob/9802945e6342ec284633924cb7d8d3ce67204995/crates/ignore/src/walk.rs#L532)],
so when traversing the file system, it would:

- Detect symlinked directories as directory entries
- But not traverse into them to index their contents
- The `get_file_path` function would then filter out actual directories,
leaving only the symlinked folder itself as a result

Fix: Added `.follow_links(true)` to the `WalkBuilder` configuration,
making the file search follow symlinks and index their contents just
like regular directories.

This change maintains backward compatibility since symlink following is
generally expected behavior for file search tools, and it aligns with
how users expect the `@` search feature to work.

Co-authored-by: Eric Traut <[email protected]>

Author: polyrand

codex-rs/file-search/src/lib.rs

@@ -159,6 +159,8 @@ pub fn run(
         .threads(num_walk_builder_threads)
         // Allow hidden entries.
         .hidden(false)
+        // Follow symlinks to search their contents.
+        .follow_links(true)
         // Don't require git to be present to apply to apply git-related ignore rules.
         .require_git(false);
     if !respect_gitignore {