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/core/tests/common/Cargo.toml

@@ -18,11 +18,16 @@ codex-utils-cargo-bin = { workspace = true }
 ctor = { workspace = true }
 futures = { workspace = true }
 notify = { workspace = true }
+opentelemetry = { workspace = true }
+opentelemetry_sdk = { workspace = true }
 regex-lite = { workspace = true }
 serde_json = { workspace = true }
 tempfile = { workspace = true }
 tokio = { workspace = true, features = ["net", "time"] }
 tokio-tungstenite = { workspace = true }
+tracing = { workspace = true }
+tracing-opentelemetry = { workspace = true }
+tracing-subscriber = { workspace = true }
 walkdir = { workspace = true }
 wiremock = { workspace = true }
 shlex = { workspace = true }