This sets the accessibility document property (AXDocument) of the
window, which other apps can use to understand what file the current
window represents. Document-based apps on macOS are generally expected
to set this property.

The document path is set via `Window::set_document_path()` which calls
`setRepresentedFilename:` on the underlying NSWindow. The workspace
updates this path in `update_window_title()` whenever the active item or
project structure changes. For tests, instead of trying to somehow
assemble a proper NSWindow, we store the document path on the window
mock used for testing and check its value.

Motivation: I am the developer of Timing, an automatic time tracking app
for Mac. Timing uses the `AXDocument` property (a standard property of
most document-based app windows on macOS) to understand what document
the user is working on. With this change, Timing is able to understand
which directory the user is working in. Without this, Timing would only
record the window title, i.e. the filename without information about the
containing directory. I've had several users ask for better support for
Zed.

Here's a screenshot of the macOS Accessibility Inspector showing the
`AXDocument` property with this change. The UI of Zed itself does not
change. However, in my dev build of Zed, the traffic lights are a bit
too large and misaligned. However, this happens even when building
`main`, so I assume it’s unrelated to my changes.

<img width="1370" height="1162" alt="Screenshot 2026-01-30 at 16 18 25"
src="https://github.com/user-attachments/assets/dc260252-91fb-41e1-97a9-e6fb843c6a70"
/>

Release Notes:

- Set the represented filename property of windows on macOS

---------

Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>