feat(youtube): multiple stream notifications

Refactors the notification storage to support multiple stream IDs
using a ConcurrentDictionary. This ensures that the bot can track and
ignore duplicate notifications for multiple streams within a
rolling 24-hour window, rather than only tracking the single most
recent stream.

- Updated ILastPostedStreamStore to include RemoveValuesOlderThan24Hours.
- Switched LastPostedStreamStore to use ConcurrentDictionary with UTC timestamps.
- Injected TimeProvider to enable deterministic testing of time-based expiration.
- Added automatic cache pruning in NotificationHandler on every request.
- Renamed Unit/LastPostedStreamStore.cs to Unit/LastPostedStreamStoreTests.cs and expanded test coverage.

src/tests/StevesBot.Webhook.Tests/Unit/LastPostedStreamStore.cs

@@ -1,36 +0,0 @@
-namespace StevesBot.Webhook.Tests.Unit;
-
-public class LastPostedStreamStoreTests
-{
-  [Fact]
-  public void SetValue_WhenCalledWithNull_ItShouldThrowArgumentNullException()
-  {
-    var store = new LastPostedStreamStore();
-
-    var act = () => store.SetValue(null!);
-
-    act.Should().Throw<ArgumentNullException>();
-  }
-
-  [Fact]
-  public void HasValue_WhenCalledWithNull_ItShouldThrowArgumentNullException()
-  {
-    var store = new LastPostedStreamStore();
-
-    var act = () => store.HasValue(null!);
-
-    act.Should().Throw<ArgumentNullException>();
-  }
-
-
-  [Fact]
-  public void SetValueHasValue_WhenCalled_ItShouldStoreValue()
-  {
-    var store = new LastPostedStreamStore();
-    var streamId = "test-stream-id";
-
-    store.SetValue(streamId);
-
-    store.HasValue(streamId).Should().BeTrue();
-  }
-}

src/tests/StevesBot.Webhook.Tests/Unit/LastPostedStreamStoreTests.cs

@@ -0,0 +1,63 @@
+namespace StevesBot.Webhook.Tests.Unit;
+
+public class LastPostedStreamStoreTests
+{
+  private readonly Mock<TimeProvider> _mockTimeProvider = new();
+  private readonly LastPostedStreamStore _sut;
+
+  public LastPostedStreamStoreTests()
+  {
+    _sut = new(_mockTimeProvider.Object);
+  }
+
+  [Fact]
+  public void SetValue_WhenCalledWithNull_ItShouldThrowArgumentNullException()
+  {
+    var act = () => _sut.SetValue(null!);
+
+    act.Should().Throw<ArgumentNullException>();
+  }
+
+  [Fact]
+  public void HasValue_WhenCalledWithNull_ItShouldThrowArgumentNullException()
+  {
+    var act = () => _sut.HasValue(null!);
+
+    act.Should().Throw<ArgumentNullException>();
+  }
+
+
+  [Fact]
+  public void SetValueHasValue_WhenCalled_ItShouldStoreValue()
+  {
+    var streamId = "test-stream-id";
+
+    _sut.SetValue(streamId);
+
+    _sut.HasValue(streamId).Should().BeTrue();
+  }
+
+  [Fact]
+  public void RemoveValuesOlderThan24Hours_WhenCalled_ItShouldRemoveItemsOlderThan24Hours()
+  {
+    var oldStreamId = "test-stream-id-old";
+    var currentStreamId = "test-stream-id-current";
+
+    var currentTime = DateTimeOffset.UtcNow;
+    var invalidTime = DateTimeOffset.UtcNow.AddHours(-25);
+    var validTime = DateTimeOffset.UtcNow.AddHours(-10);
+
+    _mockTimeProvider.SetupSequence(static m => m.GetUtcNow())
+      .Returns(invalidTime)
+      .Returns(validTime)
+      .Returns(currentTime);
+
+    _sut.SetValue(oldStreamId);
+    _sut.SetValue(currentStreamId);
+
+    _sut.RemoveValuesOlderThan24Hours();
+
+    _sut.HasValue(oldStreamId).Should().BeFalse();
+    _sut.HasValue(currentStreamId).Should().BeTrue();
+  }
+}