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/analytics/src/analytics_client_tests.rs

@@ -263,6 +263,7 @@ fn sample_thread_resume_response_with_source(
         sandbox: AppServerSandboxPolicy::DangerFullAccess,
         active_permission_profile: None,
         reasoning_effort: None,
+        initial_turns_page: None,
     })
 }
 

codex-rs/analytics/src/client_tests.rs

@@ -172,6 +172,7 @@ fn sample_thread_resume_response() -> ClientResponsePayload {
         sandbox: AppServerSandboxPolicy::DangerFullAccess,
         active_permission_profile: None,
         reasoning_effort: None,
+        initial_turns_page: None,
     })
 }