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>

ui/frame_android.py

@@ -47,7 +47,7 @@ def ssh(cmd, input=None, timeout=120):
     try:
         # No inherited stdin (see server.ssh): Windows' ssh.exe would wait on it.
         feed = {'input': input} if input is not None else {'stdin': subprocess.DEVNULL}
-        p = subprocess.run(['ssh', *SSH_OPTS, FRAME, cmd], capture_output=True, **feed,
+        p = frame_host.run_ssh(['ssh', *SSH_OPTS, FRAME, cmd], capture_output=True, **feed,
                            timeout=timeout, text=isinstance(input, str) or input is None)
     except subprocess.TimeoutExpired:
         raise FrameError(f'timed out talking to {FRAME}')
@@ -120,7 +120,7 @@ def _copy(src, dest, executable=False, timeout=600):
     else:
         cmd = ['scp', *SSH_OPTS, src, f'{FRAME}:{dest}']
     try:
-        subprocess.run(cmd, check=True, capture_output=True, stdin=subprocess.DEVNULL, text=True, timeout=timeout)
+        frame_host.run_ssh(cmd, check=True, capture_output=True, stdin=subprocess.DEVNULL, text=True, timeout=timeout)
     except subprocess.TimeoutExpired:
         raise FrameError(f'copying {name} to the Frame timed out')
     except subprocess.CalledProcessError as e: