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/Cargo.lock

@@ -2939,7 +2939,6 @@ dependencies = [
  "pretty_assertions",
  "prost",
  "rcgen",
- "reqwest 0.12.28",
  "rustls",
  "serde",
  "serde_json",
@@ -2954,6 +2953,7 @@ dependencies = [
  "tracing",
  "tracing-opentelemetry",
  "tracing-subscriber",
+ "url",
  "uuid",
  "windows-sys 0.52.0",
  "wiremock",