assistant_slash_commands: Fix AI text thread path display bugs on Windows and all platforms (#41880)

## Fix incorrect directory path folding in slash command file collection

**Description:**
This PR fixes a bug in the `collect_files` function where the directory
folding logic (used to compact chains like `.github/workflows`) failed
to reset its state when traversing out of a folded branch.

**The Issue:**
The `folded_directory_names` accumulator was persisting across loop
iterations. If the traversal moved from a folded directory (e.g.,
`.github/workflows`) to a sibling directory (e.g., `.zed`), the sibling
would incorrectly inherit the prefix of the previously folded path,
resulting in incorrect paths like `.github/.zed`.

**The Fix:**
* Introduced `folded_directory_path` to track the specific path
currently being folded.
* Added a check to reset `folded_directory_names` whenever the traversal
encounters an entry that is not a child of the currently folded path.
* Ensured state is cleared immediately after a folded directory is
rendered.

**Release Notes:**
- Fixed an issue where using slash commands to collect files would
sometimes display incorrect directory paths (e.g., showing
`.github/.zed` instead of `.zed`) when adjacent directories were
automatically folded.

---------

Co-authored-by: Lukas Wirth <lukas@zed.dev>

Cargo.lock

@@ -834,7 +834,6 @@ dependencies = [
  "fs",
  "futures 0.3.31",
  "fuzzy",
- "globset",
  "gpui",
  "html_to_markdown",
  "http_client",