[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/app-server-protocol/src/protocol/v2/tests.rs

@@ -3650,6 +3650,7 @@ fn plugin_share_list_response_serializes_share_items() {
                     installed: false,
                     enabled: false,
                     install_policy: PluginInstallPolicy::Available,
+                    install_policy_source: Some(PluginInstallPolicySource::WorkspaceSetting),
                     auth_policy: PluginAuthPolicy::OnUse,
                     availability: PluginAvailability::Available,
                     interface: None,
@@ -3672,6 +3673,7 @@ fn plugin_share_list_response_serializes_share_items() {
                     "installed": false,
                     "enabled": false,
                     "installPolicy": "AVAILABLE",
+                    "installPolicySource": "WORKSPACE_SETTING",
                     "authPolicy": "ON_USE",
                     "availability": "AVAILABLE",
                     "interface": null,