Eliminate app hang from synchronous file open

[sentry]

This PR addresses an app hang (REPOPROMPT-9D) that occurred for at least 2000 ms when opening files via NSWorkspace.shared.open().

Root Cause:
The FileViewModel.openInDefaultApp() method, and a fallback path in WorkspaceFilesViewModel.openFileForMarkdownLink(), were calling the synchronous NSWorkspace.shared.open(fileURL) on the main thread. This call blocks on a LaunchServices XPC round-trip, causing the UI to freeze.

Solution:
Replaced both instances of the synchronous NSWorkspace.shared.open(fileURL) with its asynchronous counterpart: NSWorkspace.shared.open(fileURL, configuration: NSWorkspace.OpenConfiguration(), completionHandler:). Error logging is now handled within the completion handler, preventing the main thread from blocking.

Impact of changes:

• The app no longer hangs when opening files.
• The openFileForMarkdownLink method now always returns true after dispatching the async open, as its boolean return value was previously discarded by callers (_ =), making the change in semantics practically inconsequential.

Fixes REPOPROMPT-9D