test(input): inject secure-open failures on the primitive the platform uses - #503

Problem

Two cases assert what happens when the secure, no-follow open fails. Both inject that
failure by patching os.open:

• tests/unit/test_input_handler.py::test_resolve_file_open_failure_does_not_create_temp_dir
  — "Failed secure opens leave no handler-owned temporary directory behind."
• tests/nodes/test_build_context.py::test_build_context_reports_read_error_without_fake_empty_content
  — "Unreadable files remain inventoried but are absent from the content cache."

with patch("skillspector.input_handler.os.open", side_effect=OSError("denied")):

os.open is only reached on the POSIX branch:

_HAS_SECURE_DIR_FD = os.open in os.supports_dir_fd and hasattr(os, "O_NOFOLLOW")
...
    if _HAS_SECURE_DIR_FD:
        return _open_regular_file_from_trusted_directory(absolute_path)
    if _IS_WINDOWS:
        return _open_regular_file_from_windows_handle(absolute_path)

os.supports_dir_fd is empty on Windows, so _HAS_SECURE_DIR_FD is False and the dispatcher
goes to _open_regular_file_from_windows_handle, which opens through CreateFileW and never
calls os.open. The patch therefore injects nothing: the open succeeds, the file is read, and
the assertions about the failure path never get a failure to observe.

The visible effect is that both cases fail. The substantive effect is that the failed-open
contract — no temporary directory left behind, no fabricated empty content in the cache — has
no coverage at all on the platform that uses the handle-based implementation.

Fix

Deny both primitives, so the failure reaches the dispatcher whichever branch it takes. The
Windows stub raises _FileOpenError, which is exactly what _open_regular_file_from_windows_handle
raises when CreateFileW returns INVALID_HANDLE_VALUE:

if handle == invalid_handle_value:
    error = _windows_last_error()
    if error.errno == ENOENT:
        raise FileNotFoundError(f"File not found: {file_path}") from None
    raise _FileOpenError(file_path, error)

On POSIX the added patch targets a helper that branch never calls, so behaviour there is
unchanged and the cases keep exercising the real os.open path.

Reproduction

Windows 11, Python 3.12.10, against unmodified main (704bc95):

$ python -m pytest -p no:randomly \
    "tests/unit/test_input_handler.py::test_resolve_file_open_failure_does_not_create_temp_dir" \
    "tests/nodes/test_build_context.py::test_build_context_reports_read_error_without_fake_empty_content" \
    -q --no-header

        with patch("skillspector.input_handler.os.open", side_effect=OSError("denied")):
>           with pytest.raises(ValueError, match="Could not safely open"):
E           Failed: DID NOT RAISE ValueError
tests\unit\test_input_handler.py:230: Failed

        monkeypatch.setattr("skillspector.input_handler.os.open", deny_open)
        result = build_context({"skill_path": str(tmp_path)})
        assert "broken.py" in result["components"]
>       assert "broken.py" not in result["file_cache"]
E       AssertionError: assert 'broken.py' not in {'broken.py': 'print(1)\r\n'}
tests\nodes\test_build_context.py:994: AssertionError
=========================== short test summary info ===========================
FAILED tests/unit/test_input_handler.py::test_resolve_file_open_failure_does_not_create_temp_dir
FAILED tests/nodes/test_build_context.py::test_build_context_reports_read_error_without_fake_empty_content
2 failed in 5.88s

DID NOT RAISE and the populated file_cache are the same symptom: the open was never denied.

With this change, same command:

..                                                                       [100%]
2 passed in 4.88s

They pass rather than skip, so the contract is now actually asserted on this platform.

What must still hold

Both files in full, before and after:

In both files the change converts exactly one case from failing to passing and moves nothing
else. The eleven remaining test_build_context.py failures are unrelated to secure-open
dispatch and are not touched here.

ruff check src/ tests/ — All checks passed!
ruff format --check src/ tests/ — 223 files already formatted