Support includeTurns reads for paginated threads (#36993)

## Why

Clients using `thread/read` with `includeTurns: true` need the legacy full-history
view even when a thread stores its history in paginated form.

## What changed

- Reconstruct full projected turns from paginated history for both stored and
  loaded threads.
- Keep `thread/turns/list` and `thread/items/list` as the preferred incremental
  history APIs.

## Testing

- Cover `thread/read(includeTurns=true)` before and after a paginated thread is
  loaded.

GitOrigin-RevId: 6efdc04e5ed3cad2ff2078d8103933082c7921db

codex-rs/app-server/tests/suite/v2/thread_read.rs

@@ -213,8 +213,7 @@ async fn thread_read_can_include_turns() -> Result<()> {
 }
 
 #[tokio::test]
-async fn paginated_stored_thread_routes_projected_turns_and_rejects_legacy_history_paths()
--> Result<()> {
+async fn paginated_stored_thread_routes_projected_turns() -> Result<()> {
     let server = create_mock_responses_server_repeating_assistant("Done").await;
     let codex_home = TempDir::new()?;
     MockResponsesConfig::new(&server.uri()).write(codex_home.path())?;
@@ -270,23 +269,6 @@ async fn paginated_stored_thread_routes_projected_turns_and_rejects_legacy_histo
         .expect("thread/list should include paginated thread");
     assert_eq!(listed.history_mode, ThreadHistoryMode::Paginated);
 
-    let read_id = mcp
-        .send_thread_read_request(ThreadReadParams {
-            thread_id: conversation_id.clone(),
-            include_turns: true,
-        })
-        .await?;
-    let read_err: JSONRPCError = timeout(
-        DEFAULT_READ_TIMEOUT,
-        mcp.read_stream_until_error_message(RequestId::Integer(read_id)),
-    )
-    .await??;
-    assert_eq!(read_err.error.code, -32600);
-    assert_eq!(
-        read_err.error.message,
-        "paginated threads do not support thread/read(includeTurns=true)"
-    );
-
     let turns_list_id = mcp
         .send_thread_turns_list_request(ThreadTurnsListParams {
             thread_id: conversation_id.clone(),
@@ -1512,7 +1494,7 @@ async fn thread_turns_list_rejects_unmaterialized_loaded_thread() -> Result<()>
 }
 
 #[tokio::test]
-async fn paginated_history_lists_use_projected_turns_and_items() -> Result<()> {
+async fn paginated_history_lists_and_legacy_reads_use_projected_turns_and_items() -> Result<()> {
     let server = create_mock_responses_server_repeating_assistant("Done").await;
     let codex_home = TempDir::new()?;
     MockResponsesConfig::new(&server.uri()).write(codex_home.path())?;
@@ -1663,6 +1645,17 @@ async fn paginated_history_lists_use_projected_turns_and_items() -> Result<()> {
     };
     let expected_full_turns = vec![expected_turn_1_full.clone(), expected_turn_2_full.clone()];
 
+    let read_id = mcp
+        .send_thread_read_request(ThreadReadParams {
+            thread_id: thread_id.to_string(),
+            include_turns: true,
+        })
+        .await?;
+    let ThreadReadResponse {
+        thread: unloaded_thread,
+    } = timeout(DEFAULT_READ_TIMEOUT, mcp.read_response(read_id)).await??;
+    assert_eq!(unloaded_thread.turns, expected_full_turns);
+
     let legacy_resume_id = mcp
         .send_thread_resume_request(ThreadResumeParams {
             thread_id: thread_id.to_string(),
@@ -1915,6 +1908,21 @@ async fn paginated_history_lists_use_projected_turns_and_items() -> Result<()> {
     )
     .await??;
 
+    let read_id = mcp
+        .send_thread_read_request(ThreadReadParams {
+            thread_id: thread_id.to_string(),
+            include_turns: true,
+        })
+        .await?;
+    let ThreadReadResponse {
+        thread: loaded_thread,
+    } = timeout(DEFAULT_READ_TIMEOUT, mcp.read_response(read_id)).await??;
+    assert_eq!(&loaded_thread.turns[..2], expected_full_turns);
+    assert_eq!(
+        turn_user_texts(&loaded_thread.turns),
+        vec!["continue after legacy resume"]
+    );
+
     Ok(())
 }