fix(template): use sys.stdin.readline() for snapshot confirm prompt

input() with a prompt string on some terminals (zsh + Linux Mint) echoes
^M and the prompt doesn't flush before reading, causing the prompt to
appear stuck or not accept input.

Replace with explicit sys.stdout.write()+flush() + sys.stdin.readline()
which guarantees the prompt is flushed before blocking on stdin, and
readline() + .strip() handles both \n and \r\n line endings correctly.

Tests updated to use CliRunner(input=) which populates sys.stdin inside
the runner context (monkeypatching sys.stdin.readline directly is ignored
by CliRunner which replaces sys.stdin entirely).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/python/test_commands_template.py

@@ -613,8 +613,6 @@ def test_prepare_win_snapshot_exists_skip(
     backend.list_snapshots.return_value = ["BaseClean"]
     transport = _make_transport()
     _patch_prepare(monkeypatch, backend, transport)
-    monkeypatch.setattr("builtins.input", lambda _prompt="": "N")
-
     result = CliRunner().invoke(
         cli,
         [
@@ -624,6 +622,7 @@ def test_prepare_win_snapshot_exists_skip(
             "--admin-password", "adminpass",
             "--build-password", "buildpass",
         ],
+        input="N\n",
     )
     assert result.exit_code == 0, result.output
     assert "Keeping existing snapshot" in result.output
@@ -644,8 +643,6 @@ def test_prepare_win_snapshot_exists_recreate(
     monkeypatch.setattr(tmpl_mod, "_wait_tcp", lambda *_: True)
     monkeypatch.setattr(tmpl_mod, "_wait_winrm", lambda *_: None)
     monkeypatch.setattr(tmpl_mod.time, "sleep", lambda _: None)
-    monkeypatch.setattr("builtins.input", lambda _prompt="": "Y")
-
     def _sp_run(cmd: list[str], **_: object) -> None:
         sp_calls.append(cmd)
 
@@ -660,6 +657,7 @@ def test_prepare_win_snapshot_exists_recreate(
             "--admin-password", "adminpass",
             "--build-password", "buildpass",
         ],
+        input="Y\n",
     )
     assert result.exit_code == 0, result.output
     assert any("deleteSnapshot" in " ".join(c) for c in sp_calls)