Add safety improvements and encapsulation

1. Explicitly zero out bit 63 when unpacking pointer to avoid any UB
   - Added mask: ptr_bits & 0x7FFFFFFFFFFFFFFF before ptrFromInt
2. Add ConcurrentTask.auto_delete() getter to hide implementation detail
   - The fact that auto_delete is packed into next field is now internal
   - Updated event_loop.zig to use the public getter
   - Makes the API cleaner and easier to maintain

src/bun.js/event_loop.zig

@@ -321,7 +321,7 @@ pub fn tickConcurrentWithCount(this: *EventLoop) usize {
             dest.deinit();
         }
 
-        if (task.next.autoDelete()) {
+        if (task.auto_delete()) {
             to_destroy = task;
         }
 

src/bun.js/event_loop/ConcurrentTask.zig

@@ -44,7 +44,8 @@ pub const PackedNext = packed struct(u64) {
 
     pub fn get(self: PackedNext) ?*ConcurrentTask {
         if (self.ptr_bits == 0) return null;
-        const ptr: u64 = @as(u64, self.ptr_bits);
+        // Explicitly zero out bit 63 to avoid any UB
+        const ptr: u64 = @as(u64, self.ptr_bits) & 0x7FFFFFFFFFFFFFFF;
         return @as(?*ConcurrentTask, @ptrFromInt(ptr));
     }
 
@@ -69,6 +70,12 @@ pub const Queue = bun.threading.UnboundedQueuePacked(ConcurrentTask, .next, .@"p
 pub const new = bun.TrivialNew(@This());
 pub const deinit = bun.TrivialDeinit(@This());
 
+/// Returns whether this task should be automatically deleted after completion.
+/// The auto_delete flag being stored in the next field is an implementation detail.
+pub inline fn auto_delete(this: *const ConcurrentTask) bool {
+    return this.next.autoDelete();
+}
+
 pub const AutoDeinit = enum {
     manual_deinit,
     auto_deinit,