make defaultPrompt an array, keep backcompat (#14649)

make plugins' `defaultPrompt` an array, but keep backcompat for strings.

the array is limited by app-server to 3 entries of up to 128 chars
(drops extra entries, `None`s-out ones that are too long) without
erroring if those invariants are violating.

added tests, tested locally.

codex-rs/app-server-protocol/schema/json/v2/PluginReadResponse.json

@@ -125,8 +125,12 @@
           ]
         },
         "defaultPrompt": {
+          "description": "Starter prompts for the plugin. Capped at 3 entries with a maximum of 128 characters per entry.",
+          "items": {
+            "type": "string"
+          },
           "type": [
-            "string",
+            "array",
             "null"
           ]
         },