WithMeta for McpClientTool

[PederHP]

Make it possible to send request metadata (_meta) with Tool calls, by adding a WithMeta method similar to WithProgress. The copy semantics make it easy to have (outer) host context specific metadata injections.

Usage Example:

  var tool = tools.Single(t => t.Name == "my_tool");

  // Add metadata for tracing/logging
  var toolWithMeta = tool.WithMeta(new JsonObject
  {
      ["traceId"] = correlationId,
      ["userId"] = currentUserId
  });

  // Each IChatClient can have its own metadata
  var chatClient1Tools = tools.Select(t => t.WithMeta(new JsonObject { ["clientId"] = "client-1" }));
  var chatClient2Tools = tools.Select(t => t.WithMeta(new JsonObject { ["clientId"] = "client-2" }));

This is additive, so any existing (non-McpClientTool) metadata isn't overwritten.

[Copilot]

Pull request overview

This PR adds a WithMetadata method to McpClientTool, enabling clients to include custom metadata with tool call requests. This follows the same copy-semantics pattern as the existing WithProgress method, making it easy to inject host-specific context (e.g., trace IDs, authentication tokens) into MCP tool calls without modifying the original tool instance.

Key changes:

• Added WithMetadata(JsonObject) method to McpClientTool for attaching metadata to tool calls
• Updated CallToolAsync methods to accept and properly handle metadata parameters
• Metadata is correctly merged with protocol-level fields like progress tokens when both are used

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/ModelContextProtocol.Core/Client/McpClientTool.cs	Added _metadata field, updated constructors to propagate metadata, and implemented WithMetadata method with comprehensive XML documentation
src/ModelContextProtocol.Core/Client/McpClient.Methods.cs	Updated CallToolAsync to accept metadata parameter, properly clone metadata when combining with progress tokens to prevent mutation
src/ModelContextProtocol.Core/Client/McpClientExtensions.cs	Added meta parameter to CallToolAsync extension method signature and XML documentation
tests/ModelContextProtocol.Tests/Client/McpClientToolTests.cs	Added MetadataEchoTool test server tool and four comprehensive test cases covering metadata passing, instance creation, method chaining, and multiple instances with different metadata; also cleaned up trailing whitespace

[stephentoub]

Looks like this isn't going to conflict with @mikekistler's pending change. Let's get that one in first and then rebase this one on top of it.