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/core/src/otel_init.rs

@@ -3,11 +3,11 @@ use crate::config::types::OtelExporterKind as Kind;
 use crate::config::types::OtelHttpProtocol as Protocol;
 use crate::default_client::originator;
 use crate::features::Feature;
+use codex_otel::OtelProvider;
 use codex_otel::config::OtelExporter;
 use codex_otel::config::OtelHttpProtocol;
 use codex_otel::config::OtelSettings;
 use codex_otel::config::OtelTlsConfig as OtelTlsSettings;
-use codex_otel::otel_provider::OtelProvider;
 use std::error::Error;
 
 /// Build an OpenTelemetry provider from the app Config.