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/src/protocol/v2.rs

@@ -3355,7 +3355,9 @@ pub struct PluginInterface {
     pub website_url: Option<String>,
     pub privacy_policy_url: Option<String>,
     pub terms_of_service_url: Option<String>,
-    pub default_prompt: Option<String>,
+    /// Starter prompts for the plugin. Capped at 3 entries with a maximum of
+    /// 128 characters per entry.
+    pub default_prompt: Option<Vec<String>>,
     pub brand_color: Option<String>,
     pub composer_icon: Option<AbsolutePathBuf>,
     pub logo: Option<AbsolutePathBuf>,