Declare the experimental plugin search API (#36402)

## What changed

- Add the experimental `plugin/search` request with search term, scope, working-directory, cursor, and limit parameters.
- Define paginated results that include each plugin's marketplace name and optional local marketplace path.
- Route the request through app server while returning method-not-found until the search implementation is available.

## Testing

- Add an app-server test that verifies `plugin/search` returns JSON-RPC error `-32601` with the expected not-implemented message.

GitOrigin-RevId: 755400468331f0887f7da7d933195c347aadf6d0

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

@@ -0,0 +1,47 @@
+use super::PluginSummary;
+use crate::JsonSchema;
+use crate::TS;
+use codex_utils_absolute_path::AbsolutePathBuf;
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginSearchParams {
+    pub search_term: String,
+    #[ts(optional = nullable)]
+    pub scope: Option<PluginSearchScope>,
+    #[ts(optional = nullable)]
+    pub cwds: Option<Vec<AbsolutePathBuf>>,
+    #[ts(optional = nullable)]
+    pub cursor: Option<String>,
+    #[ts(optional = nullable)]
+    pub limit: Option<u32>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(rename_all = "camelCase", export_to = "v2/")]
+pub enum PluginSearchScope {
+    Global,
+    Workspace,
+    Personal,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginSearchResult {
+    pub plugin: PluginSummary,
+    pub marketplace_name: String,
+    pub marketplace_path: Option<AbsolutePathBuf>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginSearchResponse {
+    pub data: Vec<PluginSearchResult>,
+    pub next_cursor: Option<String>,
+}