[codex] app-server: expose plugin install policy source (#31293)

## Summary

- Add `PluginInstallPolicySource` to the app-server v2 `PluginSummary`
payload.
- Propagate remote `installation_policy_source` through plugin
summaries, details, installed-plugin caching, and app-server responses.
- Preserve the supported source values and map unrecognized backend
values to `null`.
- Return `null` for local plugins and expose the field in generated JSON
and TypeScript schemas.
- Update app-server API documentation and plugin list/read coverage.

## Testing

- `just test -p codex-app-server-protocol` (251 passed)
- Targeted `codex-app-server` plugin list, installed, and read tests (4
passed)
- Targeted `codex-core-plugins` known and unknown policy source tests (2
passed)
- `just fmt`
- `git diff --check`

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

@@ -781,6 +781,7 @@ fn remote_installed_plugin_in_marketplace(
         name: name.to_string(),
         enabled: true,
         install_policy: codex_app_server_protocol::PluginInstallPolicy::Available,
+        install_policy_source: None,
         auth_policy: codex_app_server_protocol::PluginAuthPolicy::OnUse,
         availability: codex_app_server_protocol::PluginAvailability::Available,
         interface: None,