[macOS] fix: Closing tab with running process while in Stage Manager no longer causes focus to be lost (#5164)

Resolves #5108 

Reproduction: While Stage Manager is activated, create a new tab in
Ghostty, run `sleep 500` in that tab, and try and close it. Make sure to
try both closing the tab with cmd-w and clicking the x; also try
accepting the modal with a mouse click and pressing enter.

Relevant AppKit documentation: [NSAlert
beginSheetModal](https://developer.apple.com/documentation/appkit/nsalert/beginsheetmodal(for:completionhandler:))

What I believe is happening is that the alert modal ("there is still a
process running") is closed automatically after the completion handler
is run, which closes the window on which the alert is attached.
Something seems to go wrong in Stage Manager at this point; perhaps it
detects the alert closing as a "full" window closing and moves focus to
a different window.

The fix I've identified is to manually call the `orderOut` function to
dismiss the alert _before_ the code that closes the window is run.

macos/Sources/Features/Terminal/BaseTerminalController.swift

@@ -452,6 +452,7 @@ class BaseTerminalController: NSWindowController,
             self.alert = nil
             switch (response) {
             case .alertFirstButtonReturn:
+                alert.window.orderOut(nil)
                 window.close()
 
             default:

macos/Sources/Ghostty/Ghostty.TerminalSplit.swift

@@ -205,6 +205,7 @@ extension Ghostty {
             alert.beginSheetModal(for: window, completionHandler: { response in
                 switch (response) {
                 case .alertFirstButtonReturn:
+                    alert.window.orderOut(nil)
                     node = nil
 
                 default: