fix(util): read a path's folder and extension from its name

[ANIRUDDHA ADAK (aniruddhaadak80)]

What

Two helpers in tooling/util/src/path.ts work on the whole path where they should work on the filename:

export function getDirectory(path: string | undefined) {
  const trimmed = path.replace(/[\/\\]+$/, "")
  const parts = trimmed.split(/[\/\\]/)
  return parts.slice(0, parts.length - 1).join("/") + "/"
}

export function getFileExtension(path: string | undefined) {
  const parts = path.split(".")
  return parts[parts.length - 1]
}

Why it matters

A bare filename is reported as living in the filesystem root. getDirectory("foo") splits to ["foo"], the slice is [], and [].join("/") + "/" is "/":

getDirectory("foo")   -> "/"      (a relative name with no folder)
getDirectory("a/b")   -> "a/"

A file with no extension gets its own name as the extension, and a dotfile gets its name minus the dot, and a dot in a directory leaks through:

getFileExtension("README")      -> "README"     (should be "")
getFileExtension(".gitignore")  -> "gitignore"  (should be "")
getFileExtension("a.b/c")       -> "b/c"

This isn't a matter of taste — getFilenameTruncated, four lines below in the same file, already implements the correct rule for the same question (const ext = lastDot <= 0 ? "" : ...). The module contradicted itself.

It's UI-visible: session-turn.tsx:1258 renders `‪${getDirectory(diff.file)}‬` directly, so a root-level filename displays / as its directory, and any caller branching on extension misroutes extensionless and dotfile paths.

Verification

Neither helper had a test. New test/util/path-helpers.test.ts, three cases failing before:

(fail) getDirectory > a bare filename has no directory
(fail) getFileExtension > an extensionless or dotfile name has no extension
(fail) getFileExtension > reads the extension from the filename, not from a directory
 3 pass  3 fail

After:

(pass) getDirectory > a bare filename has no directory
(pass) getDirectory > keeps the directory of a nested path
(pass) getDirectory > the root and an empty input stay as they are
(pass) getFileExtension > an extensionless or dotfile name has no extension
(pass) getFileExtension > reads the extension from the filename, not from a directory
(pass) getFileExtension > an empty or missing path has no extension
 6 pass
 0 fail

The getDirectory("/") case is pinned explicitly: trimming a lone / leaves "", and that is still the root, so the new guard returns / rather than "". The whole test/util/ suite is 59 pass / 0 fail.

The change

 export function getDirectory(path: string | undefined) {
   if (!path) return ""
   const trimmed = path.replace(/[\/\\]+$/, "")
+  // A lone "/" trims away entirely, but it is still the root.
+  if (trimmed === "") return "/"
   const parts = trimmed.split(/[\/\\]/)
+  // A bare filename has no parent, and an empty slice joined to "/" would
+  // report the filesystem root instead.
+  if (parts.length <= 1) return ""
   return parts.slice(0, parts.length - 1).join("/") + "/"
 }
 
 export function getFileExtension(path: string | undefined) {
   if (!path) return ""
-  const parts = path.split(".")
-  return parts[parts.length - 1]
+  // Only the filename carries an extension: a dot in a directory name, or in
+  // an earlier path segment, is not one.
+  const base = getFilename(path)
+  const dot = base.lastIndexOf(".")
+  // A leading dot marks a dotfile, not an extension, and no dot means none.
+  if (dot <= 0) return ""
+  return base.slice(dot + 1)
 }

getFileExtension now reuses getFilename from the same module, which already strips both separators.

Touched files are Prettier-clean (verified on LF-normalized copies; this Windows checkout's core.autocrlf=true makes Prettier flag every file repo-wide).

Fixes #834

[vercel]

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