fix: add SSE heartbeat to prevent connection resets

[rynfar]

Summary

• Adds a : ping SSE comment every 15 seconds during streaming responses to keep the connection alive through reverse proxies and network intermediaries
• Wraps the SDK message loop in try/finally to guarantee heartbeat cleanup

Problem

Users running the proxy behind reverse proxies (nginx, Cloudflare, etc.) or over non-localhost networks experience connection resets during the idle period while Claude processes the request. No data flows between content_block_start and the first content_block_delta, causing intermediaries to kill the connection.

Fix

Standard SSE keepalive — periodic : comment lines that are ignored by all compliant clients but keep the TCP connection active.

Closes #1