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/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -11120,6 +11120,39 @@
       "title": "PluginReadResponse",
       "type": "object"
     },
+    "PluginSearchResult": {
+      "properties": {
+        "marketplaceName": {
+          "type": "string"
+        },
+        "marketplacePath": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "plugin": {
+          "$ref": "#/definitions/PluginSummary"
+        }
+      },
+      "required": [
+        "marketplaceName",
+        "plugin"
+      ],
+      "type": "object"
+    },
+    "PluginSearchScope": {
+      "enum": [
+        "global",
+        "workspace",
+        "personal"
+      ],
+      "type": "string"
+    },
     "PluginShareCheckoutParams": {
       "$schema": "http://json-schema.org/draft-07/schema#",
       "properties": {