[app-server] add fuzzyFileSearch/sessionCompleted (#11773)

this is to allow the client to know when to stop showing a spinner.

codex-rs/app-server-protocol/schema/json/FuzzyFileSearchSessionCompletedNotification.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "sessionId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "sessionId"
+  ],
+  "title": "FuzzyFileSearchSessionCompletedNotification",
+  "type": "object"
+}