Reuse route-aware clients for OpenAI file uploads (#35717)

## What changed

- Store a request-logging-disabled `RouteAwareClientPool` in session services and use it for file creation, blob upload, and finalization requests.
- Preserve the existing system-root fallback for transport-default proxy routes, and emit a warning event when that fallback is used.
- Remove the direct `reqwest` dependency from `codex-api` now that uploads use the shared HTTP abstraction.

## Testing

- Add coverage for completing multiple uploads through a shared client pool.

GitOrigin-RevId: 01842415cad2d349a6bd3869abb3332e8110ebce

codex-rs/http-client/src/client_builder.rs

@@ -212,6 +212,12 @@ impl HttpClientBuilder {
         match build_with_custom_ca(self.clone().reqwest_builder(proxy_routing)) {
             Ok(inner) => HttpClient::from_parts(inner, request_logging),
             Err(error) => {
+                tracing::event!(
+                    target: "codex_otel.log_only",
+                    tracing::Level::WARN,
+                    event.name = "codex.http_client.custom_ca_fallback",
+                    "HTTP client fell back to system root certificates"
+                );
                 tracing::warn!(error = %error, "failed to build HTTP client with custom CA");
                 self.reqwest_builder(proxy_routing)
                     .build()