[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 {

codex-rs/app-server-protocol/src/protocol/v2/plugin.rs

@@ -125,6 +125,19 @@ pub struct PluginListParams {
     pub marketplace_kinds: Option<Vec<PluginListMarketplaceKind>>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginInstalledParams {
+    /// Optional working directories used to discover repo marketplaces.
+    #[ts(optional = nullable)]
+    pub cwds: Option<Vec<AbsolutePathBuf>>,
+    /// Additional uninstalled plugin names that should be returned when present locally.
+    /// This is used by mention surfaces that intentionally expose install entrypoints.
+    #[ts(optional = nullable)]
+    pub install_suggestion_plugin_names: Option<Vec<String>>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
 #[ts(export_to = "v2/")]
 pub enum PluginListMarketplaceKind {
@@ -150,6 +163,15 @@ pub struct PluginListResponse {
     pub featured_plugin_ids: Vec<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginInstalledResponse {
+    pub marketplaces: Vec<PluginMarketplaceEntry>,
+    #[serde(default)]
+    pub marketplace_load_errors: Vec<MarketplaceLoadErrorInfo>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]

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

@@ -2790,6 +2790,27 @@ fn plugin_list_params_serializes_marketplace_kind_filter() {
     );
 }
 
+#[test]
+fn plugin_installed_params_serializes_install_suggestion_names() {
+    assert_eq!(
+        serde_json::to_value(PluginInstalledParams {
+            cwds: None,
+            install_suggestion_plugin_names: Some(vec![
+                "computer-use".to_string(),
+                "chrome".to_string(),
+            ]),
+        })
+        .unwrap(),
+        json!({
+            "cwds": null,
+            "installSuggestionPluginNames": [
+                "computer-use",
+                "chrome",
+            ],
+        }),
+    );
+}
+
 #[test]
 fn plugin_read_params_serialization_uses_install_source_fields() {
     let marketplace_path = if cfg!(windows) {