Upgrade MCP gem to support version 0.25

[patvice]

Summary

Upgrades the optional official MCP SDK integration to mcp ~> 0.25 while preserving RubyLLM::MCP’s native implementation and transports as an independent, first-class adapter.

Changes

• Uses official MCP::Client::Stdio and MCP::Client::HTTP transports in :mcp_sdk mode.
• Adds official lifecycle, capability negotiation, ping, pagination, completions, cache hints, structured output, metadata, extensions, OAuth, sampling, and elicitation support.
• Adds Client#sdk_client for direct access to upstream SDK functionality.
• Keeps native stdio, SSE, Streamable HTTP, WebSocket, roots, subscriptions, tasks, notifications, and human-in-the-loop behavior unchanged.
• Rejects SDK legacy SSE with actionable migration guidance.
• Updates installation, adapter, OAuth, transport, and migration documentation.
• Adds comprehensive native-isolation and SDK contract coverage.