tests: pin ink engine in _make_tui_argv npm-bootstrap tests (post-merge semantic fix)

Main's rewritten test_tui_npm_install.py tests call _make_tui_argv expecting
the Ink/npm flow unconditionally; with the dual-engine dispatch merged in,
_resolve_tui_engine() auto-selects opentui whenever ui-opentui/dist is built
in the repo, routing the call away from the path under test (first subprocess
became 'node --version' instead of 'npm run build'). Pin the engine to ink
via an autouse fixture, mirroring the existing pinning precedent in
test_tui_resume_flow.py.

tools/code_execution_tool.py

@@ -1618,6 +1618,7 @@ def _is_usable_python(python_path: str) -> bool:
             timeout=5,
             capture_output=True,
             creationflags=subprocess.CREATE_NO_WINDOW if _IS_WINDOWS else 0,
+            stdin=subprocess.DEVNULL,
         )
         return result.returncode == 0
     except (OSError, subprocess.TimeoutExpired, subprocess.SubprocessError):