fix(coderd): unarchive child chats with parents (#23761)

Unarchiving a root chat now restores descendant chats in the database
and emits lifecycle events for every affected chat so passive sessions
converge without a full refetch.

This keeps archive and unarchive symmetric at both the data and
watch-stream layers by returning the affected chat family from the
database, using those post-update rows for chatd pubsub fanout, and
covering descendant lifecycle delivery with a watch-level regression
test.

Closes #23666

coderd/database/querier_test.go

@@ -10646,7 +10646,8 @@ func TestChatPinOrderQueries(t *testing.T) {
 		}
 
 		// Archive the middle pin.
-		require.NoError(t, db.ArchiveChatByID(ctx, second.ID))
+		_, err := db.ArchiveChatByID(ctx, second.ID)
+		require.NoError(t, err)
 
 		// Archived chat should have pin_order cleared. Remaining
 		// pins keep their original positions; the next mutation