fix(mcp): use stateful transport to prevent 500 errors on Happy MCP server

[hztBUAA]

Summary

• The StreamableHTTPServerTransport in startHappyServer.ts was configured with sessionIdGenerator: undefined (stateless mode)
• In MCP SDK v1.25+, stateless mode throws "Stateless transport cannot be reused across requests" on any request after the first one
• This caused Claude Code to receive HTTP 500 errors when trying to discover tools (e.g. tools/list) after initialization, marking the Happy MCP server as "failed" and making mcp__happy__change_title unavailable
• Fix: use a fixed session ID generator so the transport operates in stateful mode, allowing the single-client local connection to handle multiple requests

Root cause analysis

1. Happy starts an HTTP MCP server on a random local port and passes it to Claude Code via --mcp-config
2. Claude Code connects and sends initialize → succeeds (first request)
3. Claude Code sends tools/list → the transport throws because stateless mode prohibits reuse → caught by the HTTP handler → returns 500
4. Claude Code marks the MCP server as "failed"
5. All mcp__happy__* tools become unavailable

Test plan

• Verified the 500 error by manually curl-ing the running MCP server
• Build and install the fixed version, start a happy session, and verify mcp__happy__change_title is available and functional

🤖 Generated with Claude Code