feat: Propagate OTel context via WebSocket HTTP upgrade headers

[lhchavez]

Why

River's WebSocket connections don't carry any OTel context (traceparent, tracestate, baggage) from client to server. This means distributed tracing and baggage propagation are broken at the WebSocket boundary — the server has no way to inherit the caller's trace context or read OTel baggage entries.

What changed

Uses the standard W3C HTTP header approach — the same mechanism any HTTP service uses for OTel propagation — applied to the WebSocket upgrade request.

Client side (client_transport.py, v2/session.py)

• Before calling websockets.connect(), inject the current OTel context into a headers dict via propagate.inject().
• Pass those headers as extra_headers (v1 legacy API) / additional_headers (v2 asyncio API) to the connect call.
• This automatically includes traceparent, tracestate, and baggage headers if the corresponding propagators are configured in the global textmap.

Server side (server.py)

• In Server.serve(), extract the OTel context from websocket.request_headers via propagate.extract().
• Attach the extracted context as the ambient OTel context for the lifetime of the connection using context.attach() / context.detach().
• Any handler code running within the connection can now read baggage via baggage.get_all() and inherits the caller's trace context.

Tests (tests/v1/test_opentelemetry.py)

• test_baggage_propagated_via_ws_headers: Sets two baggage entries on the client, verifies the server handler can read them.
• test_no_baggage_when_none_set: Verifies clean behavior when no baggage is set.
• test_traceparent_propagated_via_ws_headers: Sets both an active span and baggage on the client, verifies both propagate.

Test plan

$ uv run pytest tests/ -v
64 passed in 8.46s

All existing tests pass unchanged. The 3 new tests verify end-to-end OTel context propagation through the WebSocket connection.

Revertibility

Safe to revert — only adds new extra_headers/additional_headers to websockets.connect() and a propagate.extract() + context.attach() wrapper on the server. No wire protocol changes, no schema changes, no data mutations.

~ written by Zerg 👾