Add test validating ArgumentException for form mode elicitation without schema

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

tests/ModelContextProtocol.Tests/Protocol/UrlElicitationTests.cs

@@ -163,6 +163,26 @@ await request.Server.ElicitAsync(new()
                     throw new McpException(ex.Message);
                 }
             }
+            else if (request.Params.Name == "TestFormElicitationMissingSchema")
+            {
+                try
+                {
+                    await request.Server.ElicitAsync(new()
+                        {
+                            Message = "Form elicitation without schema should fail.",
+                        },
+                        cancellationToken);
+                }
+                catch (ArgumentException ex)
+                {
+                    throw new McpException(ex.Message);
+                }
+
+                return new CallToolResult
+                {
+                    Content = [new TextContentBlock { Text = "missing-schema-succeeded" }],
+                };
+            }
 
             Assert.Fail($"Unexpected tool name: {request.Params.Name}");
             return new CallToolResult { Content = [] };
@@ -506,6 +526,38 @@ public async Task UrlElicitationRequired_Exception_Propagates_To_Client()
         Assert.Equal("Authorization is required to access Example Co.", elicitation.Message);
     }
 
+    [Fact]
+    public async Task FormElicitation_Requires_RequestedSchema()
+    {
+        var elicitationHandlerCalled = false;
+
+        await using McpClient client = await CreateMcpClientForServer(new McpClientOptions
+        {
+            Capabilities = new ClientCapabilities
+            {
+                Elicitation = new ElicitationCapability
+                {
+                    Url = new UrlElicitationCapability(),
+                }
+            },
+            Handlers = new McpClientHandlers()
+            {
+                ElicitationHandler = (request, cancellationToken) =>
+                {
+                    elicitationHandlerCalled = true;
+                    return new ValueTask<ElicitResult>(new ElicitResult());
+                },
+            }
+        });
+
+        var result = await client.CallToolAsync("TestFormElicitationMissingSchema", cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.True(result.IsError);
+        var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
+        Assert.Equal("An error occurred invoking 'TestFormElicitationMissingSchema': Form mode elicitation requests require a requested schema.", textContent.Text);
+        Assert.False(elicitationHandlerCalled);
+    }
+
     private ElicitationCapability AssertServerElicitationCapability()
     {
         var capabilities = Server.ClientCapabilities;