[codex] Update remote connector suggestions

[ericning-o]

Summary

• Use the session-loaded plugin app IDs as the source of connector suggestion candidates.
• Remove the redundant plugin reload from tool_suggest_connector_ids().
• Add regression coverage for connectors declared by a loaded remote plugin, using the Databricks app case.

Context

Loaded remote plugins can declare app connector IDs in .app.json. The session-owned PluginsManager already loads those plugins and exposes their effective app IDs.

The connector suggestion path was creating a separate PluginsManager and recomputing plugin app IDs. That new manager does not share the session manager’s remote installed plugin cache, so app IDs from loaded remote plugins were missing from connector suggestions.

Fix

Pass the already-loaded effective app IDs into connector suggestion generation and use them directly as the plugin-derived connector candidate set.

Connector candidates are now built from:

• App IDs declared by loaded plugins
• Explicitly configured connector discoverables
• Existing disabled-suggestion filtering

This avoids a second plugin-manager lookup and keeps connector suggestions aligned with the plugins actually loaded for the turn.

Behavior

For example, when a plugin is loaded and its .app.json declares data apps, list_available_plugins_to_install can now return those data connectors.

This does not create plugin suggestions from the plugin itself. Plugin suggestions still come from eligible uninstalled entries in the marketplace catalog and require existing matching/filtering rules.

Validation

• just fmt
• Added regression coverage for a loaded-plugin connector ID appearing in discoverable tools
• Attempted just test -p codex-core; the command exited unsuccessfully in the local test environment without useful failure detail captured in the run output