Streamable HTTP resumability + redelivery + SSE polling via server-side disconnect (#1077)

Co-authored-by: Stephen Halter <halter73@gmail.com>

tests/ModelContextProtocol.Tests/Server/McpServerToolTests.cs

@@ -804,10 +804,10 @@ public partial class McpServerToolTests
     public void ReturnDescription_StructuredOutputEnabled_WithExplicitDescription_NoSynthesis()
     {
         // When UseStructuredContent is true and Description is set, return description goes to output schema
-        McpServerTool tool = McpServerTool.Create(ToolWithReturnDescription, new() 
-        { 
-            Description = "Custom description", 
-            UseStructuredContent = true 
+        McpServerTool tool = McpServerTool.Create(ToolWithReturnDescription, new()
+        {
+            Description = "Custom description",
+            UseStructuredContent = true
         });
 
         // Description should not have the return description appended
@@ -815,6 +815,23 @@ public partial class McpServerToolTests
         Assert.NotNull(tool.ProtocolTool.OutputSchema);
     }
 
+    [Fact]
+    public async Task EnablePollingAsync_ThrowsInvalidOperationException_WhenTransportIsNotStreamableHttpPost()
+    {
+        // Arrange
+        Mock<McpServer> mockServer = new();
+        var jsonRpcRequest = CreateTestJsonRpcRequest();
+
+        // The JsonRpcRequest has no Context, so RelatedTransport will be null
+        var requestContext = new RequestContext<CallToolRequestParams>(mockServer.Object, jsonRpcRequest);
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => requestContext.EnablePollingAsync(TimeSpan.FromSeconds(1), TestContext.Current.CancellationToken).AsTask());
+
+        Assert.Contains("Streamable HTTP", exception.Message);
+    }
+
     [Description("Tool that returns data.")]
     [return: Description("The computed result")]
     private static string ToolWithReturnDescription() => "result";