Add MCP 2025-11-25 tool contracts

[CountableNewt]

What changed

• Add recursive JSON arguments and JSON Schema models.
• Add structured tool results with text fallbacks and resource links.
• Add tool metadata, output schemas, annotations, icons, pagination, and JSON-RPC error data.
• Retain compatibility initializers and scalar argument access for existing consumers.

Why

MCP-5 needs native structured context, expressive schemas, and agent-readable results without duplicating protocol primitives in MyContextProtocol.

Testing

• env DEVELOPER_DIR=/Applications/Xcode-beta.app/Contents/Developer swift test
• 11 tests passed.
• git diff --check

Consumer migration

Handlers that require scalar strings should read MCPRequestParams.stringArguments; arguments now preserves recursive JSON values.