fix: change HTTP 204 to 200 for mcphost compatibility

[ckanthony]

• Replace StatusNoContent (204) with StatusOK (200) in sse.go HandleMessage
• Fixes initialization timeout issues with mcphost client
• Maintains backward compatibility with existing clients
• HTTP 200 is more semantically correct for SSE responses

Fixes #3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.95%. Comparing base (f4b2a61) to head (28c9d62).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   63.18%   63.95%   +0.76%     
==========================================
  Files           4        4              
  Lines        1081     1154      +73     
==========================================
+ Hits          683      738      +55     
- Misses        351      364      +13     
- Partials       47       52       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.