Add UseMcpClient

[jozkee]

Contributes to dotnet/extensions#6492

For IChatClient that don't have this capability, we can enable it via an McpServerChatClient : IChatClient that itself uses MCP clients. It would translate a HostedMcpServer tool into creating an McpClient and replacing the tool in the tool collection with the appropriate McpClientTool instances. With a FunctionInvokingChatClient in the pipeline, it would enable then similar automatic handling of MCP Server interactions.

cc @westey-m

[stephentoub]

Nice. Seems like this generally works out?

[PederHP]

This is a really awesome extension. ❤

[jozkee]

I added an LRU cache. Clients will be evicted if limit is reached or if the tool invocation throws an HttpRequestException, as per the spec, servers return 404 when a session is revoked, in that latter case, we immediately retry with a new client.

[jozkee]

I also chose to look for the tool by name and throw (if is not there) the McpProtocolException which is the same exception that would be thrown if the server doesn't find the tool you asked to invoke.

[jeffhandley]

Adjustment to this plan. We will reuse a single ID while still defining the consts for our own code hygiene.

Suggested change

	// public const string Tasks_DiagnosticId = "MCP5001";
	// public const string Tasks_Message = "The Tasks feature is experimental within specification version 2025-11-25 and is subject to change. See SEP-1686 for more information.";
	// public const string Tasks_Url = "https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1686";
	public const string UseMcpClient_DiagnosticId = "MCP5002";
	public const string UseMcpClient_Message = "The UseMcpClient middleware for integrating hosted MCP servers with IChatClient is experimental and subject to change.";
	// All experimental APIs share the same ID of MCP0001 but the consts define logical features
	// public const string Tasks_DiagnosticId = "MCP0001";
	// public const string Tasks_Message = "The Tasks feature is experimental within specification version 2025-11-25 and is subject to change. See SEP-1686 for more information.";
	// public const string Tasks_Url = "https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1686";
	public const string UseMcpClient_DiagnosticId = "MCP0001";
	public const string UseMcpClient_Message = "The UseMcpClient middleware for integrating hosted MCP servers with IChatClient is experimental and subject to change.";