test: stop the 1 ms promotion window leaking out of StepCacheLifecycleTests (#2679)

## Summary

`Find_TwoPathfindsOverlappingChunks_PromoteToBuildOnSecondFind` fails intermittently in CI with `second Find through overlapping chunks must promote (got 0 builds)`.

**Cause:** `SecondTouchAfterWindow_RestartsTheCount` and `TwoFindsAcrossTheWindow_DoNotPromote` set `StepCache.Instance.MissPromotionWindowMs = 1` and never restore it, and `Clear()` does not reset it. Both classes share the `Sequential Pathfinding Tests` collection, so when a lifecycle test runs first, the promotion test's two Finds have to land within 1 ms of each other. On Linux, `Environment.TickCount` moves in coarse steps of several ms, so even a sub-millisecond test can see the window lapse. The second Find then restarts the miss count instead of promoting. On Windows the ~15.6 ms tick nearly always reads the same value for both Finds, which is why it didn't reproduce locally.

**Fix (tests only):**
- `StepCacheLifecycleTests` implements `IDisposable` and restores the window after each test.
- The promotion test sets the window it depends on (30 s) instead of inheriting it.

## Test plan

- [x] Reproduced deterministically: with a temporary 20 ms sleep between the two Finds, the test fails whenever the lifecycle tests run first and passes when run alone
- [x] With the sleep still in, the test passes with the Dispose restore alone and with both fixes
- [x] All pathfinding tests pass (123/123) with the sleep removed

Projects/UOContent.Tests/Tests/Engines/Pathing/BitmapAStarAlgorithmTests.cs

@@ -371,6 +371,9 @@ public class BitmapAStarAlgorithmTests
     {
         StepCache.Instance.Clear();
         StepCache.Instance.MissPromotionThreshold = 2;
+        // Pinned: promotion needs both Finds inside the window, and Linux's coarse TickCount can
+        // step several ms between two back-to-back Finds.
+        StepCache.Instance.MissPromotionWindowMs = 30_000;
 
         var map = Map.Maps[1];
         var stub = new DefaultWalkerStub();

Projects/UOContent.Tests/Tests/Engines/Pathing/StepCacheLifecycleTests.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Reflection;
 using System.Threading;
@@ -14,8 +15,14 @@ namespace Server.Tests.Pathfinding;
 /// layers, and LRU eviction.
 /// </summary>
 [Collection("Sequential Pathfinding Tests")]
-public class StepCacheLifecycleTests
+public class StepCacheLifecycleTests : IDisposable
 {
+    // The window is a knob on the singleton and Clear() leaves it alone, so a test that narrows
+    // it would otherwise leak the narrow window into every later test in the collection.
+    private readonly uint _savedWindowMs = StepCache.Instance.MissPromotionWindowMs;
+
+    public void Dispose() => StepCache.Instance.MissPromotionWindowMs = _savedWindowMs;
+
     /// <summary>Resets to a known state and returns the singleton.</summary>
     private static StepCache FreshCache(int promotionThreshold)
     {