fix(happy-cli): use stateful StreamableHTTPServerTransport to prevent 500 errors

[Wshid]

Summary

The MCP server in startHappyServer.ts uses StreamableHTTPServerTransport with sessionIdGenerator: undefined (stateless mode). Since MCP SDK @modelcontextprotocol/sdk >= 1.27.x, stateless transports throw "Stateless transport cannot be reused across requests" after the first request.

Because startHappyServer creates a single transport and reuses it for all incoming HTTP requests, every request after the initial initialize silently returns HTTP 500. This breaks the change_title MCP tool — Claude Code cannot rename chat sessions.

Fix

Switch from stateless to stateful mode:

```diff

• sessionIdGenerator: undefined

• sessionIdGenerator: () => randomUUID()
  ```

randomUUID is already imported from node:crypto in the same file.

How to reproduce

1. Start a happy session (happy)
2. Claude Code launches with --mcp-config {"mcpServers":{"happy":{"type":"http","url":"http://127.0.0.1:{port}/"}}}
3. Try to call mcp__happy__change_title → fails with 500
4. Directly test: curl -X POST http://127.0.0.1:{port}/ -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}},"id":1}' → HTTP 500

Tested

• yarn build ✅
• yarn typecheck ✅
• yarn test — 1 pre-existing failure in runAcp.test.ts (unrelated log format mismatch)
• Manual verification: after patch, new happy sessions can successfully call change_title