Change systemd KillMode from process to mixed for better process cleanup

Co-authored-by: salmanmkc <32169182+salmanmkc@users.noreply.github.com>

src/Test/L0/Misc/SystemdServiceTemplateL0.cs

@@ -0,0 +1,48 @@
+using System;
+using System.IO;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Misc
+{
+    public sealed class SystemdServiceTemplateL0
+    {
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void ServiceTemplate_ContainsExpectedKillMode()
+        {
+            // Arrange
+            var templatePath = Path.Combine(TestUtil.GetSrcPath(), "Misc", "layoutbin", "actions.runner.service.template");
+            
+            // Act
+            var templateContent = File.ReadAllText(templatePath);
+            
+            // Assert
+            Assert.Contains("KillMode=mixed", templateContent);
+            Assert.Contains("KillSignal=SIGTERM", templateContent);
+            Assert.Contains("TimeoutStopSec=5min", templateContent);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void ServiceTemplate_HasValidStructure()
+        {
+            // Arrange
+            var templatePath = Path.Combine(TestUtil.GetSrcPath(), "Misc", "layoutbin", "actions.runner.service.template");
+            
+            // Act
+            var templateContent = File.ReadAllText(templatePath);
+            var lines = templateContent.Split('\n', StringSplitOptions.RemoveEmptyEntries);
+            
+            // Assert
+            Assert.Contains("[Unit]", lines);
+            Assert.Contains("[Service]", lines);
+            Assert.Contains("[Install]", lines);
+            Assert.Contains("Description={{Description}}", lines);
+            Assert.Contains("ExecStart={{RunnerRoot}}/runsvc.sh", lines);
+            Assert.Contains("User={{User}}", lines);
+            Assert.Contains("WorkingDirectory={{RunnerRoot}}", lines);
+        }
+    }
+}