[codex] Add installed-plugin mention API (#22448)

## Summary
- add app-server `plugin/installed` for mention-oriented plugin loading
- return installed plugins plus explicitly requested install-suggestion
rows
- keep remote handling on installed-state data instead of the broad
catalog listing path

## Why
The `@` mention surface only needs plugins that are usable now, plus a
small product-approved set of install suggestions. It does not need the
full catalog-shaped `plugin/list` payload that the Plugins page uses.

## Validation
- `just write-app-server-schema`
- `just fmt`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-core-plugins`
- `cargo test -p codex-app-server --test all plugin_installed_`

## Notes
- The package-wide `cargo test -p codex-app-server` run still hits an
existing unrelated stack overflow in
`in_process::tests::in_process_start_clamps_zero_channel_capacity`.
- Companion webview PR: https://github.com/openai/openai/pull/915672

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -625,6 +625,11 @@ client_request_definitions! {
         serialization: None,
         response: v2::PluginListResponse,
     },
+    PluginInstalled => "plugin/installed" {
+        params: v2::PluginInstalledParams,
+        serialization: None,
+        response: v2::PluginInstalledResponse,
+    },
     PluginRead => "plugin/read" {
         params: v2::PluginReadParams,
         serialization: None,
@@ -1719,6 +1724,15 @@ mod tests {
         };
         assert_eq!(plugin_read.serialization_scope(), None);
 
+        let plugin_installed = ClientRequest::PluginInstalled {
+            request_id: request_id(),
+            params: v2::PluginInstalledParams {
+                cwds: None,
+                install_suggestion_plugin_names: None,
+            },
+        };
+        assert_eq!(plugin_installed.serialization_scope(), None);
+
         let plugin_uninstall = ClientRequest::PluginUninstall {
             request_id: request_id(),
             params: v2::PluginUninstallParams {