Add public constructors to McpClient types for reusing cached definitions (#938)

* Initial plan

* Add public constructor to McpClientTool for reusing tool definitions

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

* Fix McpClientTool constructor per review feedback

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

* Add public constructors for McpClientPrompt, McpClientResource, and McpClientResourceTemplate

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

tests/ModelContextProtocol.Tests/Client/McpClientResourceTemplateConstructorTests.cs

@@ -0,0 +1,103 @@
+using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol;
+using ModelContextProtocol.Server;
+using System.ComponentModel;
+
+namespace ModelContextProtocol.Tests.Client;
+
+public class McpClientResourceTemplateConstructorTests : ClientServerTestBase
+{
+    public McpClientResourceTemplateConstructorTests(ITestOutputHelper outputHelper)
+        : base(outputHelper)
+    {
+    }
+
+    protected override void ConfigureServices(ServiceCollection services, IMcpServerBuilder mcpServerBuilder)
+    {
+        mcpServerBuilder.WithResources<FileTemplateResources>();
+    }
+
+    [McpServerResourceType]
+    private sealed class FileTemplateResources
+    {
+        [McpServerResource, Description("A file template")]
+        public static string FileTemplate([Description("The file path")] string path) => $"Content for {path}";
+    }
+
+    [Fact]
+    public async Task Constructor_WithValidParameters_CreatesInstance()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var templates = await client.ListResourceTemplatesAsync(cancellationToken: TestContext.Current.CancellationToken);
+        var originalTemplate = templates.First();
+        var templateDefinition = originalTemplate.ProtocolResourceTemplate;
+
+        var newTemplate = new McpClientResourceTemplate(client, templateDefinition);
+
+        Assert.NotNull(newTemplate);
+        Assert.Equal("file_template", newTemplate.Name);
+        Assert.Equal("A file template", newTemplate.Description);
+        Assert.Same(templateDefinition, newTemplate.ProtocolResourceTemplate);
+    }
+
+    [Fact]
+    public void Constructor_WithNullClient_ThrowsArgumentNullException()
+    {
+        var templateDefinition = new ResourceTemplate
+        {
+            UriTemplate = "file:///{path}",
+            Name = "test",
+            Description = "Test template"
+        };
+
+        Assert.Throws<ArgumentNullException>("client", () => new McpClientResourceTemplate(null!, templateDefinition));
+    }
+
+    [Fact]
+    public async Task Constructor_WithNullResourceTemplate_ThrowsArgumentNullException()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        Assert.Throws<ArgumentNullException>("resourceTemplate", () => new McpClientResourceTemplate(client, null!));
+    }
+
+    [Fact]
+    public async Task ReuseResourceTemplateDefinition_PreservesTemplateMetadata()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+        
+        var templates = await client.ListResourceTemplatesAsync(cancellationToken: TestContext.Current.CancellationToken);
+        var originalTemplate = templates.First();
+        var templateDefinition = originalTemplate.ProtocolResourceTemplate;
+
+        var reusedTemplate = new McpClientResourceTemplate(client, templateDefinition);
+
+        Assert.Equal(originalTemplate.Name, reusedTemplate.Name);
+        Assert.Equal(originalTemplate.Description, reusedTemplate.Description);
+        Assert.Equal(originalTemplate.UriTemplate, reusedTemplate.UriTemplate);
+        Assert.Equal(originalTemplate.ProtocolResourceTemplate.Name, reusedTemplate.ProtocolResourceTemplate.Name);
+        Assert.Equal(originalTemplate.ProtocolResourceTemplate.Description, reusedTemplate.ProtocolResourceTemplate.Description);
+    }
+
+    [Fact]
+    public async Task ManuallyConstructedResourceTemplate_CreatesValidInstance()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var manualTemplate = new ResourceTemplate
+        {
+            UriTemplate = "file:///{path}",
+            Name = "file_template",
+            Description = "A file template"
+        };
+
+        var clientTemplate = new McpClientResourceTemplate(client, manualTemplate);
+
+        Assert.NotNull(clientTemplate);
+        Assert.Equal("file_template", clientTemplate.Name);
+        Assert.Equal("A file template", clientTemplate.Description);
+        Assert.Equal("file:///{path}", clientTemplate.UriTemplate);
+    }
+}