Adding L0 tests for Legacy NodeHandler logic | Adding 2 Knobs for feature flag & EOL toggle status value

src/Agent.Sdk/Knob/AgentKnobs.cs

@@ -568,6 +568,13 @@ public class AgentKnobs
             new EnvironmentKnobSource("AGENT_DISABLE_NODE6_TASKS"),
             new BuiltInDefaultKnobSource("false"));
 
+        public static readonly Knob EnableEOLNodeVersionPolicy = new Knob(
+            nameof(EnableEOLNodeVersionPolicy),
+            "When enabled, automatically upgrades tasks using end-of-life Node.js versions (6, 10, 16) to supported versions (Node 20.1 or Node 24). Throws error if no supported versions are available on the agent.",
+            new PipelineFeatureSource("AGENT_RESTRICT_EOL_NODE_VERSIONS"),
+            new EnvironmentKnobSource("AGENT_RESTRICT_EOL_NODE_VERSIONS"),
+            new BuiltInDefaultKnobSource("false"));
+
         public static readonly Knob DisableTeePluginRemoval = new Knob(
             nameof(DisableTeePluginRemoval),
             "Disables removing TEE plugin after using it during checkout.",
@@ -929,5 +936,12 @@ public class AgentKnobs
             new PipelineFeatureSource("EnableDockerExecDiagnostics"),
             new EnvironmentKnobSource("AGENT_ENABLE_DOCKER_EXEC_DIAGNOSTICS"),
             new BuiltInDefaultKnobSource("false"));
+
+        public static readonly Knob UseNodeVersionStrategy = new Knob(
+            nameof(UseNodeVersionStrategy),
+            "If true, use the strategy pattern for Node.js version selection (both host and container). This provides centralized node selection logic with EOL policy enforcement. Set to false to use legacy node selection logic.",
+            new PipelineFeatureSource("UseNodeVersionStrategy"),
+            new EnvironmentKnobSource("AGENT_USE_NODE_STRATEGY"),
+            new BuiltInDefaultKnobSource("false"));
     }
 }

src/Test/L0/NodeHandlerCollections.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using Xunit;
+
+namespace Microsoft.VisualStudio.Services.Agent.Tests
+{
+    /// <summary>
+    /// Single collection for ALL NodeHandler tests (legacy and unified).
+    /// This ensures sequential execution to prevent environment variable conflicts.
+    /// </summary>
+    [CollectionDefinition("Unified NodeHandler Tests")]
+    public class UnifiedNodeHandlerTestFixture : ICollectionFixture<UnifiedNodeHandlerTestFixture>
+    {
+        // This class is never instantiated, it's just a collection marker
+    }
+}

src/Test/L0/NodeHandlerL0.AllSpecs.cs

@@ -0,0 +1,32 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.Linq;
+using Xunit;
+
+namespace Microsoft.VisualStudio.Services.Agent.Tests
+{
+    /// <summary>
+    /// Unified test runner for ALL NodeHandler test specifications.
+    /// Executes every scenario defined in NodeHandlerTestSpecs.AllScenarios.
+    /// </summary>
+    [Trait("Level", "L0")]
+    [Trait("Category", "NodeHandler")]
+    [Collection("Unified NodeHandler Tests")]
+    public sealed class NodeHandlerL0AllSpecs : NodeHandlerTestBase
+    {
+        [Theory]
+        [MemberData(nameof(GetAllNodeHandlerScenarios))]
+        public void NodeHandler_AllScenarios(TestScenario scenario)
+        {
+            RunScenarioAndAssert(scenario);
+        }
+
+        public static object[][] GetAllNodeHandlerScenarios()
+        {
+            return NodeHandlerTestSpecs.AllScenarios
+                .Select(scenario => new object[] { scenario })
+                .ToArray();
+        }
+    }
+}