fix(explorer): mounted directories being detected as non-directories in Tree:expand (#2053)

## Description

In certain filesystems — such as bind mounts, NFS, FUSE, or other
mounted paths — uv.fs_scandir_next() may return a nil type for valid
directories. This causes the snacks.nvim explorer to treat mounted
folders as files, making them unexpandable in the UI.

This patch adds a fallback to resolve the correct type when
fs_scandir_next() returns nil. It uses uv.fs_stat() first, and falls
back to vim.fn.isdirectory() if needed.

This ensures that mounted directories are properly marked as navigable.

Code change summary:

- Inside Tree:expand, we now check t == nil
- If so, we call uv.fs_stat() to get the true type
- If fs_stat fails, we fallback to vim.fn.isdirectory
- Final dir = true logic remains intact

Impact:
- Fixes a long-standing bug in mounted directories under /mnt, FUSE,
etc.
- Only runs extra checks when needed (no performance hit for regular
files)
- No changes elsewhere — clean, isolated patch

## Related Issue(s)
  - Fixes #2036

## Screenshots

before:
<img width="1907" height="946" alt="image"
src="https://github.com/user-attachments/assets/15e1b8a5-c999-49e8-8ab5-1d23c60e4969"
/>

after:
<img width="343" height="902" alt="Screenshot_20250715_222920"
src="https://github.com/user-attachments/assets/c7ca2fa0-e0cc-4e3d-a4bb-5345538408f7"
/>

---------

Co-authored-by: Folke Lemaitre <folke.lemaitre@gmail.com>

lua/snacks/explorer/tree.lua

@@ -141,6 +141,7 @@ function Tree:expand(node)
     if not name then
       break
     end
+    t = t or Snacks.util.path_type(node.path .. "/" .. name)
     found[name] = true
     local child = self:child(node, name, t)
     child.type = t

lua/snacks/picker/preview.lua

@@ -13,6 +13,7 @@ function M.directory(ctx)
   ctx.preview:set_title(ctx.item.title or name)
   local ls = {} ---@type {file:string, type:"file"|"directory"}[]
   for file, t in vim.fs.dir(ctx.item.file) do
+    t = t or Snacks.util.path_type(ctx.item.file .. "/" .. file)
     ls[#ls + 1] = { file = file, type = t }
   end
   ctx.preview:set_lines(vim.split(string.rep("\n", #ls), "\n"))

lua/snacks/picker/util/init.lua

@@ -454,7 +454,7 @@ function M.dir(item)
   local path = type(item) == "table" and M.path(item) or item
   ---@cast path string
   path = svim.fs.normalize(path)
-  return vim.fn.isdirectory(path) == 1 and path or vim.fs.dirname(path)
+  return Snacks.util.path_type(path) == "directory" and path or vim.fs.dirname(path)
 end
 
 ---@param paths string[]
@@ -476,7 +476,7 @@ function M.copy_path(from, to)
     Snacks.notify.error(("File `%s` does not exist"):format(from))
     return
   end
-  if vim.fn.isdirectory(from) == 1 then
+  if Snacks.util.path_type(from) == "directory" then
     M.copy_dir(from, to)
   else
     M.copy_file(from, to)

lua/snacks/util/init.lua

@@ -458,4 +458,18 @@ function M.parse(parser, range, on_parse)
   end
 end
 
+--- Better validation to check if path is a dir or a file
+---@param path string
+---@return "directory"|"file"
+function M.path_type(path)
+  local stat = uv.fs_stat(path)
+  if stat and stat.type then
+    return stat.type
+  end
+  if vim.fn.isdirectory(path) == 1 then
+    return "directory"
+  end
+  return "file"
+end
+
 return M