fix: ClearAllTimers stops every timer in a slot, not just the first (#2723)

## Summary
`Timer.ClearAllTimers` walked each wheel slot with a `do/while` that read `node._nextTimer` but never advanced `node`. `Stop()` unlinks the head from its slot, so the second pass read a null successor from the same, now-detached node and exited. Only the first timer in every slot was stopped; the rest kept running.

The loop now reads the successor, stops the node, and advances.

Nothing in the repo calls `ClearAllTimers` today, so no running server is affected. It is public API for shards and tests.

## Tests
- `TimerTests.ClearAllTimers_StopsEveryTimerInASlot` puts three timers in the same slot, clears them, asserts none is `Running`, and slices past their due time. It fails without the fix.
- Server.Tests: all pass.

Projects/Server/Timer/Timer.TimerWheel.cs

@@ -321,13 +321,14 @@ public partial class Timer
             for (var i = 0; i < _ringSize; i++)
             {
                 var node = t[i];
-                Timer next;
 
-                do
+                while (node != null)
                 {
-                    next = node?._nextTimer;
-                    node?.Stop();
-                } while (next != null);
+                    // Stop() unlinks the node, so read its successor first.
+                    var next = node._nextTimer;
+                    node.Stop();
+                    node = next;
+                }
             }
         }
     }