feat: add X-Accel-Buffering header to SSE streaming responses - #10078

Summary

All AgentOS SSE streaming endpoints (agent / team / workflow runs, A2A streaming, AG-UI) now send the X-Accel-Buffering: no response header, so reverse proxies such as nginx stream events through instead of buffering them.

Without this header, a default nginx proxy_pass deployment buffers the upstream response: clients receive streamed events in delayed bursts, or only after the whole run completes. FastAPI/Starlette flushes every chunk correctly — the buffering happens at the proxy layer, and this header is nginx's documented per-response opt-out. It is inert for direct connections and ignored by proxies that don't recognize it.

The header is added directly at each StreamingResponse construction site (30 sites across os/routers and os/interfaces), so there is no middleware overhead on the hot streaming path.

Issue number: #10077

Type of change

• Bug fix
• New feature
• Breaking change
• Improvement
• Model update
• Other:

Checklist

• Code complies with style guidelines
• Ran format/validation scripts (./scripts/format.sh and ./scripts/validate.sh)
• Self-review completed
• Documentation updated (comments, docstrings)
• Examples and guides: Relevant cookbook examples have been included or updated (if applicable)
• Tested in clean environment
• Tests added/updated (if applicable)

Additional Notes

./scripts/validate.sh reports 3 mypy errors in libs/agno/agno/models/anthropic/claude.py and libs/agno/agno/models/aws/claude.py; these exist on main and are unrelated to this change. No new public API surface is added, so no docstring or cookbook updates are required.