[Schema][SEP-1319] Decouple Request Payload from RPC Methods Definition

Issue to address and track SEP-1319:

This SEP proposes a structural refactoring of the Model Context Protocol (MCP) specification. The core change is to define payload of requests (e.g., CallToolRequest) as independent definitions and have the RPC method definitions refer to these models. This decouples the definition of the data payload from the definition of the remote procedure that transports it, leading to a clearer, more modular, and more maintainable specification.

Spec Changes: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1692/files

SEP-1319 とリンクされている仕様変更から始め、次に PHP SDK のリクエスト payload と RPC メソッド定義を追跡します。payload 定義が独立し、RPC メソッド定義が SDK 全体で一貫してそれらを参照していれば完了です。