### Overview
This PR:
- Updates `app-server-test-client` to load OTEL settings from
`$CODEX_HOME/config.toml` and initializes its own OTEL provider.
- Add real client root spans to app-server test client traces.
This updates `codex-app-server-test-client` so its Datadog traces
reflect the full client-driven flow instead of a set of server spans
stitched together under a synthetic parent.
Before this change, the test client generated a fake `traceparent` once
and reused it for every JSON-RPC request. That kept the requests in one
trace, but there was no real client span at the top, so Datadog ended up
showing the sequence in a slightly misleading way, where all RPCs were
anchored under `initialize`.
Now the test client:
- loads OTEL settings from the normal Codex config path, including
`$CODEX_HOME/config.toml` and existing --config overrides
- initializes tracing the same way other Codex binaries do when trace
export is enabled
- creates a real client root span for each scripted command
- creates per-request client spans for JSON-RPC methods like
`initialize`, `thread/start`, and `turn/start`
- injects W3C trace context from the current client span into
request.trace instead of reusing a fabricated carrier
This gives us a cleaner trace shape in Datadog:
- one trace URL for the whole scripted flow
- a visible client root span
- proper client/server parent-child relationships for each app-server
request
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