Ensure tab state is torn down on window close (#3364)

Task/Issue URL:
https://app.asana.com/0/1201048563534612/1208463834186044/f

**Description**:

For some weeks we've had reports about music playing after a browser
window playing music is closed. Turns out an indirect reference to the
WebView was being held through the `ActiveDomainPublisher` introduced
during #3076. I've made
an attempt at fixing this, but lemme know if there's a better way.

**Steps to test this PR**:
1. Open a single browser window.
2. Go to soundcloud.com and start any music playing
3. Close the window
**4. Music should stop playing**

**Definition of Done**:

* [ ] Does this PR satisfy our [Definition of
Done](https://app.asana.com/0/1202500774821704/1207634633537039/f)?

---
###### Internal references:
[Pull Request Review
Checklist](https://app.asana.com/0/1202500774821704/1203764234894239/f)
[Software Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552)
[Technical Design
Template](https://app.asana.com/0/59792373528535/184709971311943)
[Pull Request
Documentation](https://app.asana.com/0/1202500774821704/1204012835277482/f)

DuckDuckGo/NetworkProtection/AppTargets/BothAppTargets/ActiveDomainPublisher.swift

@@ -29,6 +29,7 @@ final class ActiveDomainPublisher {
     private var activeWindowControllerCancellable: AnyCancellable?
     private var activeTabViewModelCancellable: AnyCancellable?
     private var activeTabContentCancellable: AnyCancellable?
+    private var unregisterWindowControllerCancellable: AnyCancellable?
 
     @MainActor private weak var activeWindowController: MainWindowController? {
         didSet {
@@ -47,6 +48,7 @@ final class ActiveDomainPublisher {
 
         Task { @MainActor in
             subscribeToKeyWindowControllerChanges()
+            subscribeToUnregisteringWindowController()
         }
     }
 
@@ -76,6 +78,16 @@ final class ActiveDomainPublisher {
             .assign(to: \.activeDomain, onWeaklyHeld: self)
     }
 
+    @MainActor
+    private func subscribeToUnregisteringWindowController() {
+        unregisterWindowControllerCancellable = windowControllersManager.didUnregisterWindowController.sink { [weak self] in
+            guard let self else { return }
+            if self.activeWindowController == $0 {
+                self.activeWindowController = nil
+            }
+        }
+    }
+
     private func domain(from tabContent: Tab.TabContent) -> String? {
         if case .url(let url, _, _) = tabContent {