fix: Deduplicate installed local and remote curated plugins (#25681)

## Summary
- Deduplicate installed `openai-curated` and `openai-curated-remote`
plugin conflicts by feature flag.
- Prefer remote when remote plugins are enabled; otherwise prefer local,
while preserving one-sided installs.

## Testing
- `just fmt`
- `git diff --check`
- Targeted `just test` was blocked locally because `cargo-nextest` is
not installed.

codex-rs/core-plugins/src/manager.rs

@@ -492,6 +492,7 @@ impl PluginsManager {
             self.remote_installed_plugin_configs(),
             &self.store,
             self.restriction_product,
+            config.remote_plugin_enabled,
         )
         .await;
         log_plugin_load_errors(&outcome);
@@ -537,6 +538,7 @@ impl PluginsManager {
             self.remote_installed_plugin_configs(),
             &self.store,
             self.restriction_product,
+            config.remote_plugin_enabled,
         )
         .await
     }