Call resetContextGroup on page removal

Calling it here ensures that the inspector gets reset on internal page
navigation. We were seeing intermittent segfaults on a problematic WPT tests
(/encoding/legacy-mb-japanese/euc-jp/) which I believe this solves.

(The tests are still broken. Because we don't support form targets, they cause
the root page to reload in a tight cycle, causing a lot of context creation /
destruction, which I thin was the issue. This commit doesn't fix the broken test
but it hopefully fixes the crash).

Also, clear out the Inspector's default_context when the default context is
destroyed. (This was the first thing I did to try to fix the crash, it didn't
work, but I believe it's correct).

src/cdp/cdp.zig

@@ -459,6 +459,12 @@ pub fn BrowserContext(comptime CDP_T: type) type {
             }
             self.isolated_worlds.clearRetainingCapacity();
 
+            // do this before closeSession, since we don't want to process any
+            // new notification (Or maybe, instead of the deinit above, we just
+            // rely on those notifications to do our normal cleanup?)
+
+            self.notification.unregisterAll(self);
+
             // If the session has a page, we need to clear it first. The page
             // context is always nested inside of the isolated world context,
             // so we need to shutdown the page one first.
@@ -466,7 +472,6 @@ pub fn BrowserContext(comptime CDP_T: type) type {
 
             self.node_registry.deinit();
             self.node_search_list.deinit();
-            self.notification.unregisterAll(self);
             self.notification.deinit();
 
             if (self.http_proxy_changed) {