feat(core, tracing): create turn spans over websockets (#14632)

## Description

Dependent on:
- [responsesapi] https://github.com/openai/openai/pull/760991 
- [codex-backend] https://github.com/openai/openai/pull/760985

`codex app-server -> codex-backend -> responsesapi` now reuses a
persistent websocket connection across many turns. This PR updates
tracing when using websockets so that each `response.create` websocket
request propagates the current tracing context, so we can get a holistic
end-to-end trace for each turn.

Tracing is propagated via special keys (`ws_request_header_traceparent`,
`ws_request_header_tracestate`) set in the `client_metadata` param in
Responses API.

Currently tracing on websockets is a bit broken because we only set
tracing context on ws connection time, so it's detached from a
`turn/start` request.

codex-rs/Cargo.lock

@@ -3086,6 +3086,8 @@ dependencies = [
  "ctor 0.6.3",
  "futures",
  "notify",
+ "opentelemetry",
+ "opentelemetry_sdk",
  "pretty_assertions",
  "regex-lite",
  "reqwest",
@@ -3094,6 +3096,9 @@ dependencies = [
  "tempfile",
  "tokio",
  "tokio-tungstenite",
+ "tracing",
+ "tracing-opentelemetry",
+ "tracing-subscriber",
  "walkdir",
  "wiremock",
  "zstd",