[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/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/")]