As a platform engineer, I want to manage MCP session IDs and store per-session metadata in memory for now, with a future option to plug in an external store, so that session-aware routing and inspection is possible.

Considerations:

• In-memory map initially
• Design a clear abstraction for session storage so external backends (e.g. Redis) can be added later
• removal of sessions when a backend/upstream mcp server responds with a 404 (session id not found/invalid/expired) - see https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#session-management
• session inspection for observability/auditing

Upstream issue https://github.com/david-martin/mcp-helper/issues/5