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/otel/README.md

@@ -2,8 +2,8 @@
 
 `codex-otel` is the OpenTelemetry integration crate for Codex. It provides:
 
-- Provider wiring for log/trace/metric exporters (`codex_otel::OtelProvider`,
-  `codex_otel::provider`, and the compatibility shim `codex_otel::otel_provider`).
+- Provider wiring for log/trace/metric exporters (`codex_otel::OtelProvider`
+  and `codex_otel::provider`).
 - Session-scoped business event emission via `codex_otel::SessionTelemetry`.
 - Low-level metrics APIs via `codex_otel::metrics`.
 - Trace-context helpers via `codex_otel::trace_context` and crate-root re-exports.