Honor system proxy settings for remote plugins (#34509)

## Why

Remote plugin requests did not use Codex's effective outbound proxy policy, so
they could bypass configured system proxy and PAC routing.

## What changed

- Route remote plugin catalog, mutation, sharing, upload, and bundle download
  requests through the configured route-aware HTTP client.
- Select routes using the complete request URL, including encoded query
  parameters and backend-provided signed upload or download URLs.
- Preserve standard Codex headers and suppress diagnostics for URLs or headers
  that may contain credentials.

## Testing

Add coverage for route selection of catalog queries, workspace plugin uploads,
and backend-provided bundle download URLs.

GitOrigin-RevId: 106936659e21decf145b6ab4d4be84992e386861

codex-rs/app-server/src/request_processors/plugins.rs

@@ -1583,6 +1583,7 @@ impl PluginRequestProcessor {
         })?;
 
         let result = codex_core_plugins::remote_bundle::download_and_install_remote_plugin_bundle(
+            &remote_plugin_service_config,
             config.codex_home.to_path_buf(),
             validated_bundle,
         )