tests: add tests for ephemeral cache control model

tests/AnthropicClient.Tests/Unit/Models/EphemeralCacheControlTests.cs

@@ -0,0 +1,10 @@
+namespace AnthropicClient.Tests.Unit.Models;
+
+public class EphemeralCacheControlTests
+{
+  [Fact]
+  public void Constructor_WhenCalled_ItShouldInitializeType()
+  {
+    new EphemeralCacheControl().Type.Should().Be(CacheControlType.Ephemeral);
+  }
+}