test: add unflushed ostream redirect regression

Cover the buffered-before-move case for `scoped_ostream_redirect`, which still crashes despite the current move fix. This gives the PR a direct reproducer for the remaining bug path.

Made-with: Cursor

Author: rwgk

tests/test_iostream.cpp

@@ -131,6 +131,15 @@ TEST_SUBMODULE(iostream, m) {
         std::cout << msg_after << std::flush;
     });
 
+    m.def("move_redirect_output_unflushed",
+          [](const std::string &msg_before, const std::string &msg_after) {
+              py::scoped_ostream_redirect redir1(std::cout,
+                                                 py::module_::import("sys").attr("stdout"));
+              std::cout << msg_before;
+              py::scoped_ostream_redirect redir2(std::move(redir1));
+              std::cout << msg_after << std::flush;
+          });
+
     // Redirect a stream whose original rdbuf is nullptr, then move the redirect.
     // Verifies that nullptr is correctly restored (not confused with a moved-from sentinel).
     m.def("move_redirect_null_rdbuf", [](const std::string &msg) {

tests/test_iostream.py

@@ -291,6 +291,13 @@ def test_move_redirect(capsys):
     assert not stderr
 
 
+def test_move_redirect_unflushed(capsys):
+    m.move_redirect_output_unflushed("before_move", "after_move")
+    stdout, stderr = capsys.readouterr()
+    assert stdout == "before_moveafter_move"
+    assert not stderr
+
+
 def test_move_redirect_null_rdbuf(capsys):
     m.move_redirect_null_rdbuf("hello")
     stdout, stderr = capsys.readouterr()