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.Tests/Tests/Timer/TimerTests.cs

@@ -85,6 +85,30 @@ public class TimerTests
         Assert.Equal(2, timerTicks.ExecutedCount);
     }
 
+    [Fact]
+    public void ClearAllTimers_StopsEveryTimerInASlot()
+    {
+        Timer.Init(0);
+
+        var timers = new Timer[3];
+        for (var i = 0; i < timers.Length; i++)
+        {
+            timers[i] = Timer.DelayCall(TimeSpan.FromMilliseconds(80), () => Assert.Fail("cleared timer fired"));
+        }
+
+        Timer.ClearAllTimers(0);
+
+        for (var i = 0; i < timers.Length; i++)
+        {
+            Assert.False(timers[i].Running);
+        }
+
+        for (var tick = 8; tick <= 160; tick += 8)
+        {
+            Timer.Slice(tick);
+        }
+    }
+
     private class TimerTicks
     {
         public long ExpectedTicks;