Implements the MCP 2024-11-05 SSE transport directly in the ST plugin
using only stdlib (no pip, no npx required). Agents connect via:
  { "type": "sse", "url": "http://127.0.0.1:9502/sse" }  (Windows)
  { "type": "sse", "url": "http://127.0.0.1:9503/sse" }  (Mac/Linux)

Also adds tests/test_mcp_sse.py (68 integration tests for the SSE layer).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>