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/McpClientResourceTests.cs

@@ -0,0 +1,128 @@
+using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol;
+using ModelContextProtocol.Server;
+using System.ComponentModel;
+
+namespace ModelContextProtocol.Tests.Client;
+
+public class McpClientResourceTests : ClientServerTestBase
+{
+    public McpClientResourceTests(ITestOutputHelper outputHelper)
+        : base(outputHelper)
+    {
+    }
+
+    protected override void ConfigureServices(ServiceCollection services, IMcpServerBuilder mcpServerBuilder)
+    {
+        mcpServerBuilder.WithResources<SampleResources>();
+    }
+
+    [McpServerResourceType]
+    private sealed class SampleResources
+    {
+        [McpServerResource, Description("A sample resource")]
+        public static string Sample() => "Sample content";
+    }
+
+    [Fact]
+    public async Task Constructor_WithValidParameters_CreatesInstance()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var resources = await client.ListResourcesAsync(cancellationToken: TestContext.Current.CancellationToken);
+        var originalResource = resources.First();
+        var resourceDefinition = originalResource.ProtocolResource;
+
+        var newResource = new McpClientResource(client, resourceDefinition);
+
+        Assert.NotNull(newResource);
+        Assert.Equal("sample", newResource.Name);
+        Assert.Equal("A sample resource", newResource.Description);
+        Assert.Same(resourceDefinition, newResource.ProtocolResource);
+    }
+
+    [Fact]
+    public void Constructor_WithNullClient_ThrowsArgumentNullException()
+    {
+        var resourceDefinition = new Resource
+        {
+            Uri = "file:///test.txt",
+            Name = "test",
+            Description = "Test resource"
+        };
+
+        Assert.Throws<ArgumentNullException>("client", () => new McpClientResource(null!, resourceDefinition));
+    }
+
+    [Fact]
+    public async Task Constructor_WithNullResource_ThrowsArgumentNullException()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        Assert.Throws<ArgumentNullException>("resource", () => new McpClientResource(client, null!));
+    }
+
+    [Fact]
+    public async Task ReuseResourceDefinition_AcrossDifferentClients_ReadsSuccessfully()
+    {
+        Resource resourceDefinition;
+        {
+            await using McpClient client1 = await CreateMcpClientForServer();
+            var resources = await client1.ListResourcesAsync(cancellationToken: TestContext.Current.CancellationToken);
+            var sampleResource = resources.First(r => r.Name == "sample");
+            resourceDefinition = sampleResource.ProtocolResource;
+        }
+
+        await using McpClient client2 = await CreateMcpClientForServer();
+
+        var reusedResource = new McpClientResource(client2, resourceDefinition);
+
+        var result = await reusedResource.ReadAsync(
+            cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.NotNull(result);
+        Assert.NotNull(result.Contents);
+        var content = result.Contents.FirstOrDefault() as TextResourceContents;
+        Assert.NotNull(content);
+        Assert.Equal("Sample content", content.Text);
+    }
+
+    [Fact]
+    public async Task ReuseResourceDefinition_PreservesResourceMetadata()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+        
+        var resources = await client.ListResourcesAsync(cancellationToken: TestContext.Current.CancellationToken);
+        var originalResource = resources.First();
+        var resourceDefinition = originalResource.ProtocolResource;
+
+        var reusedResource = new McpClientResource(client, resourceDefinition);
+
+        Assert.Equal(originalResource.Name, reusedResource.Name);
+        Assert.Equal(originalResource.Description, reusedResource.Description);
+        Assert.Equal(originalResource.Uri, reusedResource.Uri);
+        Assert.Equal(originalResource.ProtocolResource.Name, reusedResource.ProtocolResource.Name);
+        Assert.Equal(originalResource.ProtocolResource.Description, reusedResource.ProtocolResource.Description);
+    }
+
+    [Fact]
+    public async Task ManuallyConstructedResource_CreatesValidInstance()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var manualResource = new Resource
+        {
+            Uri = "file:///sample.txt",
+            Name = "sample",
+            Description = "A sample resource"
+        };
+
+        var clientResource = new McpClientResource(client, manualResource);
+
+        Assert.NotNull(clientResource);
+        Assert.Equal("sample", clientResource.Name);
+        Assert.Equal("A sample resource", clientResource.Description);
+        Assert.Equal("file:///sample.txt", clientResource.Uri);
+    }
+}