Added new test class PromptInstanceTests

Author: AndrewMorgan1

tests/PromptStream.AI.Tests/Models/PromptInstanceTests.cs

@@ -0,0 +1,47 @@
+using PromptStream.AI.Models;
+using Xunit;
+
+namespace PromptStream.AI.Tests.Models
+{
+    public class PromptInstanceTests
+    {
+        [Fact]
+        public void Constructor_ShouldThrowArgumentNullException_WhenTemplateIsNull()
+        {
+            var vars = new Dictionary<string, string>();
+            var ex = Assert.Throws<ArgumentNullException>(() => new PromptInstance(null!, vars, "Hello"));
+            Assert.Equal("template", ex.ParamName);
+        }
+
+        [Fact]
+        public void Constructor_ShouldThrowArgumentNullException_WhenVariablesIsNull()
+        {
+            var template = new PromptTemplate { Template = "Hi" };
+            var ex = Assert.Throws<ArgumentNullException>(() => new PromptInstance(template, null!, "Hello"));
+            Assert.Equal("variables", ex.ParamName);
+        }
+
+        [Fact]
+        public void Constructor_ShouldThrowArgumentNullException_WhenRenderedTextIsNull()
+        {
+            var template = new PromptTemplate { Template = "Hi" };
+            var vars = new Dictionary<string, string>();
+            var ex = Assert.Throws<ArgumentNullException>(() => new PromptInstance(template, vars, null!));
+            Assert.Equal("renderedText", ex.ParamName);
+        }
+
+        [Fact]
+        public void Constructor_ShouldCreateInstance_WhenAllArgumentsValid()
+        {
+            var template = new PromptTemplate { Template = "Hi" };
+            var vars = new Dictionary<string, string> { ["name"] = "Andrew" };
+
+            var instance = new PromptInstance(template, vars, "Hi Andrew");
+
+            Assert.Equal(template, instance.Template);
+            Assert.Equal(vars, instance.Variables);
+            Assert.Equal("Hi Andrew", instance.RenderedText);
+            Assert.True(instance.CreatedUtc <= DateTime.UtcNow);
+        }
+    }
+}