fix(server): proxy websocket sandbox routes

[liuxiaopai-ai]

Summary

• add server-side websocket proxy routes for /sandboxes/{id}/proxy/{port} and nested paths
• support proxying the root path returned by use_server_proxy=true
• forward endpoint routing headers through both HTTP and websocket proxy paths

Root cause

Issue #383 is real. The server proxy only registered HTTP routes and explicitly treated websocket upgrades as unsupported, so browser websocket handshakes never reached the sandbox backend. Separately, get_endpoint(..., use_server_proxy=true) returned /proxy/{port} but the server only handled /proxy/{port}/{full_path}.

Testing

• cd server && uv run pytest tests/test_routes_proxy.py
• cd server && uv run pytest tests/test_routes*.py
• cd server && uv run pyright src/api/lifecycle.py tests/test_routes_proxy.py
• cd server && uv run ruff check src/api/lifecycle.py tests/test_routes_proxy.py

Fixes #383

[liuxiaopai-ai]

@ninan-nn @Pangjiping This should fix issue #383. Could you please take a look when you have a moment? Thanks!

[Pangjiping]

@ninan-nn @Pangjiping This should fix issue #383. Could you please take a look when you have a moment? Thanks!

Thanks for this, I will review this feature.