Add task calls for MCP client tools

[luisangelrod]

Summary

• expose the owning client and a reusable request-parameter builder from McpClientTool
• add CallAsTaskAsync and CallWithPollingAsync extensions for discovered client tools
• preserve the original protocol tool name, serializer settings, and merged WithMeta / RequestOptions metadata
• document direct task calls from McpClientTool

The tool extensions intentionally do not add get/cancel/update wrappers that accept arbitrary task IDs; callers can continue using the existing McpClient lifecycle methods for those operations.

Validation

• McpClientTaskMethodsTests: 12 passed
• McpClientToolTests: 39 passed
• Tasks package builds for net10.0, net9.0, net8.0, and netstandard2.0 with 0 warnings
• dotnet format --verify-no-changes passes

Closes #1730