feat(mcp): add HTTP/SSE transport support

[joshrotenberg]

Summary

Add HTTP transport support to the MCP server, in addition to the existing stdio transport.

Parent tracking issue: joshrotenberg/jpx-cloud#1 (jpx-server as multi-agent query service)

Motivation

• Enables remote MCP server deployment (not just local stdio)
• Allows multiple clients to connect to a single server instance
• Better suited for containerized/cloud deployments
• Foundation for SaaS offering

Current State

• Using rmcp v0.12 with transport-io feature (stdio only)
• rmcp already supports HTTP via transport-streamable-http-server feature
• Latest rmcp is v0.14.0

Proposed Implementation

1. Update rmcp to latest version

2. Add feature flag in Cargo.toml:

   mcp-http = ["rmcp/transport-streamable-http-server", ...]

3. Add CLI flags via clap:

   # Current behavior (default)
   jpx-server --transport stdio
   
   # HTTP mode
   jpx-server --transport http --port 3000
   jpx-server --transport http --port 3000 --host 0.0.0.0

4. Transport selection in mcp/mod.rs:

   match args.transport {
       Transport::Stdio => service.serve(stdio()).await?,
       Transport::Http => service.serve(StreamableHttpService::new(...)).await?,
   }

Open Questions

1. Authentication: Should we support API keys in this PR or separate? (Leaning separate - keep this focused on transport)
2. TLS: Support HTTPS directly or assume reverse proxy handles it initially?
3. Default port: 3000? 8080? Something else?

References

• MCP Transport Specification
• rmcp docs