feat(app-server): include turns page on thread resume (#23534)

## Summary

The client currently calls `thread/resume` to establish live updates and
immediately follows it with `thread/turns/list` to hydrate recent turns.
This lets `thread/resume` return that page directly, eliminating a round
trip and the ordering/deduplication gap between the two calls.

Experimental clients opt in with `initialTurnsPage: { limit,
sortDirection, itemsView }`. The response returns `initialTurnsPage` as
a `TurnsPage`, including cursors for paging further back in history.
Keeping the controls in a nested opt-in object provides the useful
`thread/turns/list` knobs without spreading page-specific parameters
across `thread/resume`.

## Verification

- `just fmt`
- `just write-app-server-schema --experimental`
- `just write-app-server-schema`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server
thread_resume_initial_turns_page_matches_requested_turns_list_page
--tests`
- `cargo test -p codex-app-server
thread_resume_rejoins_running_thread_even_with_override_mismatch
--tests`
- `just fix -p codex-app-server-protocol -p codex-app-server`

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

@@ -397,6 +397,11 @@ pub struct ThreadResumeParams {
     #[experimental("thread/resume.excludeTurns")]
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub exclude_turns: bool,
+    /// When present, include a `thread/turns/list` page in the resume response
+    /// so clients can bootstrap recent turns without a second request.
+    #[experimental("thread/resume.initialTurnsPage")]
+    #[ts(optional = nullable)]
+    pub initial_turns_page: Option<ThreadResumeInitialTurnsPageParams>,
     /// Deprecated and ignored by app-server. Kept only so older clients can
     /// continue sending the field while rollout persistence always uses the
     /// limited history policy.
@@ -435,6 +440,44 @@ pub struct ThreadResumeResponse {
     #[serde(default)]
     pub active_permission_profile: Option<ActivePermissionProfile>,
     pub reasoning_effort: Option<ReasoningEffort>,
+    /// `thread/turns/list` page returned when requested by `initialTurnsPage`.
+    #[experimental("thread/resume.initialTurnsPage")]
+    #[serde(default)]
+    pub initial_turns_page: Option<TurnsPage>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadResumeInitialTurnsPageParams {
+    /// Optional turn page size.
+    #[ts(optional = nullable)]
+    pub limit: Option<u32>,
+    /// Optional turn pagination direction; defaults to descending.
+    #[ts(optional = nullable)]
+    pub sort_direction: Option<SortDirection>,
+    /// How much item detail to include for each returned turn; defaults to summary.
+    #[ts(optional = nullable)]
+    pub items_view: Option<TurnItemsView>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct TurnsPage {
+    pub data: Vec<Turn>,
+    pub next_cursor: Option<String>,
+    pub backwards_cursor: Option<String>,
+}
+
+impl From<ThreadTurnsListResponse> for TurnsPage {
+    fn from(response: ThreadTurnsListResponse) -> Self {
+        Self {
+            data: response.data,
+            next_cursor: response.next_cursor,
+            backwards_cursor: response.backwards_cursor,
+        }
+    }
 }
 
 #[derive(