Fix TCP half-close propagation for relay clients

[elikoga]

Summary

• Negotiate TCP half-close support between access clients, relay, and edge agents.
• Propagate access-client stdin EOF to the target socket with write_eof() while preserving the reverse data stream.
• Add an integration regression test that requires the access client to exit cleanly after stdin EOF.
• Update the example runner to use FastAPI's current WebSocket route API.

Verification

• uv run pytest -q
• 4 passed