Add guarded LFU LRU integrity checks to soak tests

[Copilot]

The LFU soak integrity check did not verify that each internal LRU list's stored count matched its actual contents. This adds count-vs-enumeration validation for the window, protected, and probation lists, with a guard to fail fast if list corruption or enumerator bugs cause non-terminating traversal.

• What changed

  • Added an enumeration-based integrity assertion for each LFU LRU segment:
    • window
    • protected
    • probation
  • Compared the number of enumerated nodes to the list’s tracked Count.
  • Added an enumeration guard limit so corrupted list links or broken enumeration cannot loop indefinitely during soak validation.

• Where

  • BitFaster.Caching.UnitTests/Lfu/ConcurrentLfuSoakTests.cs
  • Extended ConcurrentLfuIntegrityChecker to assert list integrity before existing dictionary/list cross-checks.

• Failure mode covered

  • Detects mismatched LRU counts.
  • Detects corrupted linked-list traversal that would otherwise over-enumerate or never terminate.
  • Surfaces corruption in the specific segment (window, protected, or probation) with targeted assertion messages.

• Example

  AssertLruCountMatchesEnumeration(this.windowLru, "window");
  AssertLruCountMatchesEnumeration(this.protectedLru, "protected");
  AssertLruCountMatchesEnumeration(this.probationLru, "probation");

  private void AssertLruCountMatchesEnumeration(LfuNodeList<K, V> lfuNodes, string lruName)
  {
      int enumeratedCount = 0;
      int enumerationGuardLimit = Math.Max(this.cache.Capacity, this.dictionary.Count) + 1;
  
      foreach (var node in lfuNodes)
      {
          enumeratedCount++;
          enumeratedCount.Should().BeLessThanOrEqualTo(
              enumerationGuardLimit,
              $"{lruName} LRU enumeration exceeded the guard limit");
      }
  
      enumeratedCount.Should().Be(lfuNodes.Count, $"{lruName} LRU has a corrupted count");
  }

[coveralls]

coverage: 99.136%. remained the same — copilot/add-integrity-check-lru-lists into main