Plug entrypoint for the MCP Streamable HTTP transport.

The plug is configured by MCPKit.Router and expects:

• a host definition module implementing MCPKit.Definition
• a persistent session store returned by that definition
• a compile-time registry of declared tools for the mounted scope

Current runtime scope is intentionally narrow:

• initialize
• GET session stream over SSE
• notifications/initialized
• ping
• completion/complete
• prompts/list
• prompts/get
• resources/list
• resources/read
• resources/templates/list
• tools/list
• tools/call
• notifications/cancelled
• DELETE session termination

The plug now also uses request-time policy checks for tools, prompts, and resources via the host's configured MCPKit.Policy module.