Decouple exec-server HTTP from reqwest types (#35059)

## What changed

- Rename `ReqwestHttpClient` to `RouteAwareHttpClient` to reflect that delegated HTTP uses Codex's shared route-aware transport.
- Use `codex_http_client` response and error types plus transport-neutral `http` and `url` types, removing the exec server's direct `reqwest` dependency.

## Testing

- Cover fragment stripping and Unicode hostname normalization for delegated HTTP requests.

GitOrigin-RevId: 8b0fc60a76004feb57198bfb4afb1371c9ceb1bd

codex-rs/exec-server/src/trace_context.rs

@@ -1,5 +1,5 @@
-use reqwest::header::HeaderMap;
-use reqwest::header::HeaderValue;
+use http::HeaderMap;
+use http::HeaderValue;
 
 pub(crate) fn current_trace_context_headers() -> HeaderMap {
     let mut headers = HeaderMap::new();