feat(app-server-test-client): OTEL setup for tracing (#13493)

### 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

codex-rs/cli/src/main.rs

@@ -473,11 +473,12 @@ fn run_execpolicycheck(cmd: ExecPolicyCheckCommand) -> anyhow::Result<()> {
     cmd.run()
 }
 
-fn run_debug_app_server_command(cmd: DebugAppServerCommand) -> anyhow::Result<()> {
+async fn run_debug_app_server_command(cmd: DebugAppServerCommand) -> anyhow::Result<()> {
     match cmd.subcommand {
         DebugAppServerSubcommand::SendMessageV2(cmd) => {
             let codex_bin = std::env::current_exe()?;
             codex_app_server_test_client::send_message_v2(&codex_bin, &[], cmd.user_message, &None)
+                .await
         }
     }
 }
@@ -755,7 +756,7 @@ async fn cli_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
         },
         Some(Subcommand::Debug(DebugCommand { subcommand })) => match subcommand {
             DebugSubcommand::AppServer(cmd) => {
-                run_debug_app_server_command(cmd)?;
+                run_debug_app_server_command(cmd).await?;
             }
             DebugSubcommand::ClearMemories => {
                 run_debug_clear_memories_command(&root_config_overrides, &interactive).await?;