[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/remote/share/tests.rs

@@ -648,6 +648,7 @@ async fn list_remote_plugin_shares_fetches_created_workspace_plugins() {
                     installed: false,
                     enabled: false,
                     install_policy: PluginInstallPolicy::Available,
+                    install_policy_source: None,
                     auth_policy: PluginAuthPolicy::OnUse,
                     availability: PluginAvailability::Available,
                     interface: Some(expected_plugin_interface()),
@@ -687,6 +688,7 @@ async fn list_remote_plugin_shares_fetches_created_workspace_plugins() {
                     installed: true,
                     enabled: true,
                     install_policy: PluginInstallPolicy::Available,
+                    install_policy_source: None,
                     auth_policy: PluginAuthPolicy::OnUse,
                     availability: PluginAvailability::Available,
                     interface: Some(expected_plugin_interface()),