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/deny.toml

@@ -242,7 +242,6 @@ deny = [
         "codex-api",
         "codex-app-server",
         "codex-core",
-        "codex-exec-server",
         "codex-ollama",
         "codex-otel",
         "codex-protocol",