Fix Windows OpenSSH stderr capture in app and tests

Windows OpenSSH 9.5 blocks while writing captured stderr to a pipe, even with stdin disconnected and a connection timeout. Capture stderr in a temporary file for one-shot OpenSSH calls on Windows, preserving subprocess output, text, check, and timeout behavior. Leave POSIX capture unchanged.

Use the shared runner for SSH, scp, key lookup, and streamed app-data transfers. Bound the real ssh-keygen hashing tests and keep their assertions; move the transfer-error mock to the runner seam. Add ten regression tests.

Verified the full suite on Windows 11 with bundled Python 3.12.14: 628 tests, OK (110 existing skips), 42.685s. Verified macOS Python 3.9.6: 628 tests, OK, 67.934s. Independent Codex gpt-6-sol high-reasoning review found no actionable issues. Protected RDP code is unchanged.

Co-Authored-By: GPT-6.1 Sol (Codex) <noreply@openai.com>

tests/test_frame_host.py

@@ -0,0 +1,87 @@
+"""Captured OpenSSH output keeps working on Windows and POSIX hosts."""
+import sandbox  # noqa: F401
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+from unittest import mock
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent / "ui"))
+import frame_host
+
+
+class CapturedSSH(unittest.TestCase):
+    def run_command(self, source, **kwargs):
+        with mock.patch.object(frame_host, "WINDOWS", True):
+            return frame_host.run_ssh([sys.executable, "-c", source], timeout=5, **kwargs)
+
+    def test_binary_output_and_input(self):
+        result = self.run_command("import sys; sys.stdout.buffer.write(sys.stdin.buffer.read()); "
+                                  "sys.stderr.buffer.write(b'error\\r\\n')",
+                                  capture_output=True, input=b"data\x00\xff")
+        self.assertEqual(result.stdout, b"data\x00\xff")
+        self.assertEqual(result.stderr, b"error\r\n")
+
+    def test_text_output_normalizes_newlines(self):
+        result = self.run_command("import sys; sys.stdout.write(sys.stdin.read()); "
+                                  "sys.stderr.buffer.write(b'first\\r\\nsecond\\rthird\\n')",
+                                  capture_output=True, input="hello\n", text=True)
+        self.assertEqual(result.stdout, "hello\n")
+        self.assertEqual(result.stderr, "first\nsecond\nthird\n")
+
+    def test_explicit_encoding_and_errors(self):
+        result = self.run_command("import sys; sys.stderr.buffer.write(b'\\xe9\\xff')",
+                                  capture_output=True, encoding="ascii", errors="replace")
+        self.assertEqual(result.stderr, "\ufffd\ufffd")
+
+    def test_check_preserves_error_output(self):
+        with self.assertRaises(subprocess.CalledProcessError) as caught:
+            self.run_command("import sys; print('out'); print('err', file=sys.stderr); sys.exit(7)",
+                             capture_output=True, text=True, check=True)
+        self.assertEqual(caught.exception.returncode, 7)
+        self.assertEqual(caught.exception.stdout, "out\n")
+        self.assertEqual(caught.exception.stderr, "err\n")
+
+    def test_timeout_preserves_partial_stderr(self):
+        with self.assertRaises(subprocess.TimeoutExpired) as caught:
+            with mock.patch.object(frame_host, "WINDOWS", True):
+                frame_host.run_ssh([sys.executable, "-c", "import sys, time; "
+                                    "sys.stderr.write('waiting'); sys.stderr.flush(); time.sleep(10)"],
+                                   capture_output=True, text=True, timeout=1)
+        self.assertEqual(caught.exception.stderr, b"waiting")
+
+    def test_streamed_stdout_is_kept_separate(self):
+        with tempfile.TemporaryFile() as output:
+            result = self.run_command("import sys; sys.stdout.buffer.write(b'file'); "
+                                      "sys.stderr.buffer.write(b'error')",
+                                      stdout=output, stderr=subprocess.PIPE)
+            output.seek(0)
+            self.assertEqual(output.read(), b"file")
+        self.assertIsNone(result.stdout)
+        self.assertEqual(result.stderr, b"error")
+
+    def test_uncaptured_windows_call_is_unchanged(self):
+        with mock.patch.object(frame_host, "WINDOWS", True), mock.patch.object(subprocess, "run") as run:
+            frame_host.run_ssh(["ssh", "-V"], stderr=subprocess.DEVNULL, timeout=5)
+        run.assert_called_once_with(["ssh", "-V"], stderr=subprocess.DEVNULL, timeout=5)
+
+    def test_posix_call_is_unchanged(self):
+        with mock.patch.object(frame_host, "WINDOWS", False), mock.patch.object(subprocess, "run") as run:
+            frame_host.run_ssh(["ssh", "-V"], capture_output=True, check=True, timeout=5)
+        run.assert_called_once_with(["ssh", "-V"], capture_output=True, check=True, timeout=5)
+
+    def test_capture_rejects_explicit_streams(self):
+        for stream in ("stdout", "stderr"):
+            with self.subTest(stream=stream), self.assertRaises(ValueError):
+                self.run_command("", capture_output=True, **{stream: subprocess.DEVNULL})
+
+    @unittest.skipUnless(shutil.which("ssh"), "needs OpenSSH")
+    def test_real_ssh_failure_returns_stderr_without_hanging(self):
+        result = frame_host.run_ssh(["ssh", "-F", os.devnull, "-o", "BatchMode=yes",
+                                    "-o", "ConnectTimeout=2", "frame-control-test.invalid", "true"],
+                                   capture_output=True, stdin=subprocess.DEVNULL, text=True, timeout=5)
+        self.assertEqual(result.returncode, 255)
+        self.assertIn("Could not resolve hostname", result.stderr)