Fix CFNetwork SIGABRT crash when creating download tasks on invalidated session

The background URLSession could be in an invalid state when downloadTask(with:)
is called, because invalidateAndCancel() is asynchronous internally. This adds
an ObjC exception handler to catch NSExceptions from CFNetwork, nil guards on
the session, and safer session lifecycle management (nil after invalidation,
finishTasksAndInvalidate for cellular toggle).

Yattee/Helpers/ObjCExceptionHandler.m

@@ -0,0 +1,20 @@
+//
+//  ObjCExceptionHandler.m
+//  Yattee
+//
+//  Catches ObjC NSExceptions that Swift cannot handle natively.
+//
+
+#import "ObjCExceptionHandler.h"
+
+BOOL tryCatchObjCException(void (NS_NOESCAPE ^block)(void), NSException *_Nullable *_Nullable outException) {
+    @try {
+        block();
+        return YES;
+    } @catch (NSException *exception) {
+        if (outException) {
+            *outException = exception;
+        }
+        return NO;
+    }
+}