app-server: Add ephemeral field to Thread object (#13084)

Currently there is no alternative way to know that thread is ephemeral,
only client which did create it has the knowledge.

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

@@ -2517,6 +2517,8 @@ pub struct Thread {
     pub id: String,
     /// Usually the first user message in the thread, if available.
     pub preview: String,
+    /// Whether the thread is ephemeral and should not be materialized on disk.
+    pub ephemeral: bool,
     /// Model provider used for this thread (for example, 'openai').
     pub model_provider: String,
     /// Unix timestamp (in seconds) when the thread was created.