fix(mcp-http): keep streamable sessions alive on GET

[yamachu]

Summary

• Keep StreamableHTTP sessions alive by refreshing access time on GET /mcp when a session ID is provided.
• Return 404 for missing StreamableHTTP sessions to avoid SSE fallback and session churn.
• Reduces repeated reconnect loops observed with MCP client tools.

Reproduction

• Start the MCP server in HTTP (streamable) mode.
• Connect from an MCP client(e.g. n8n AI Agent MCP Client) that uses StreamableHTTP.
• Wait 5–10 minutes to confirm the session does not churn.
• Observe repeated GET /mcp calls and frequent session churn.