fix(otel): make HTTP trace export survive app-server runtimes (#14300)

## Summary

This PR fixes OTLP HTTP trace export in runtimes where the previous
exporter setup was unreliable, especially around app-server usage. It
also removes the old `codex_otel::otel_provider` compatibility shim and
switches remaining call sites over to the crate-root
`codex_otel::OtelProvider` export.

## What changed

- Use a runtime-safe OTLP HTTP trace exporter path for Tokio runtimes.
- Add an async HTTP client path for trace export when we are already
inside a multi-thread Tokio runtime.
- Make provider shutdown flush traces before tearing down the tracer
provider.
- Add loopback coverage that verifies traces are actually sent to
`/v1/traces`:
  - outside Tokio
  - inside a multi-thread Tokio runtime
  - inside a current-thread Tokio runtime
- Remove the `codex_otel::otel_provider` shim and update remaining
imports.

## Why

I hit cases where spans were being created correctly but never made it
to the collector. The issue turned out to be in exporter/runtime
behavior rather than the span plumbing itself. This PR narrows that gap
and gives us regression coverage for the actual export path.

codex-rs/app-server/src/lib.rs

@@ -513,7 +513,6 @@ pub async fn run_main_with_transport(
         .map(|layer| layer.with_filter(Targets::new().with_default(Level::TRACE)));
     let otel_logger_layer = otel.as_ref().and_then(|o| o.logger_layer());
     let otel_tracing_layer = otel.as_ref().and_then(|o| o.tracing_layer());
-
     let _ = tracing_subscriber::registry()
         .with(stderr_fmt)
         .with(feedback_layer)
@@ -826,6 +825,10 @@ pub async fn run_main_with_transport(
         let _ = handle.await;
     }
 
+    if let Some(otel) = otel {
+        otel.shutdown();
+    }
+
     Ok(())
 }