Default durable threads to paginated history (#40677)

## What changed

When `thread/start` omits `historyMode`, select `paginated` for durable
threads whose store supports `thread/turns/list` and `thread/items/list`.
Continue to default ephemeral threads and threads backed by stores without
those APIs to `legacy`.

Explicit `legacy` and `paginated` selections keep their existing behavior.

## Testing

- Verify durable threads use paginated history with the local store.
- Verify ephemeral threads and stores without history-list support use legacy
  history.

GitOrigin-RevId: c94b6a3b522049d541da33c41618c4142370f1a7

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -1415,6 +1415,10 @@ impl ThreadRequestProcessor {
                 DynamicToolSpec::Namespace(namespace) => namespace.tools.len(),
             })
             .sum();
+        let history_mode = history_mode.or_else(|| {
+            (!config.ephemeral && thread_store.supports_paginated_history_lists())
+                .then_some(ThreadHistoryMode::Paginated)
+        });
         let mut thread_extension_init = ExtensionDataInit::new();
         if !selected_capability_roots.is_empty() {
             thread_extension_init.insert(selected_capability_roots);