Add unit tests for streamable HTTP SSE handling

[SoldierSacha]

Summary

• add focused unit tests that exercise streamable HTTP SSE event handling, GET stream processing, resumption requests, and SSE response completion logic
• mark Windows-specific fixtures and permission checks with coverage pragmas so platform skips no longer reduce coverage

Testing

• PYTHONPATH=src python -m pytest tests/shared/test_streamable_http_unit.py -q

---

Codex Task