fix: mock subprocess in test_missing_nvidia_smi_warning instead of relying on host hardware

[MLuqmanBR]

Problem

TestGracefulDegradation::test_missing_nvidia_smi_warning fails on any
machine that has nvidia-smi installed (e.g. Linux with an NVIDIA GPU),
because it calls _nvidia_query with no mock and asserts the result is
"N/A". The stale inline comment reveals the assumption baked in:

# On macOS (where these tests actually run), nvidia-smi doesn't exist
assert result == "N/A"

This violates the module's own stated contract — the file docstring reads:

"100% line coverage target with NO real subprocess calls, GPU, or
llama.cpp binaries. All platform-specific probes are mocked.
Tests pass on macOS AND Linux."

The test makes a real subprocess call, depends on host hardware, and
only passes on macOS. It fails on Linux with an NVIDIA GPU even when
the installation is correct.

Root Cause

A duplicate of this test already exists and is implemented correctly:
TestBackgroundMonitor::test_graceful_na_when_probes_fail uses
@patch("subprocess.check_output", side_effect=FileNotFoundError) to
simulate a missing binary. test_missing_nvidia_smi_warning is the same
test without the mock.

Fix

Add the same @patch decorator that the rest of the test suite uses for
_nvidia_query, and add a mock_subp parameter to receive it. Remove
the now-incorrect inline comment.

Changes

tests/test_turbo_hardware_diag.py — TestGracefulDegradation:

# Before
def test_missing_nvidia_smi_warning(self, tmp_path):
    result = thd.BackgroundMonitor._nvidia_query("temperature.gpu")
    # On macOS (where these tests actually run), nvidia-smi doesn't exist
    assert result == "N/A"

# After
@patch("subprocess.check_output", side_effect=FileNotFoundError)
def test_missing_nvidia_smi_warning(self, mock_subp, tmp_path):
    result = thd.BackgroundMonitor._nvidia_query("temperature.gpu")
    assert result == "N/A"

Testing

• 557/557 pass on Linux with an NVIDIA GPU present (nvidia-smi returns
  real temperature data).
• The mock correctly triggers the FileNotFoundError path that
  _nvidia_query already handles for graceful degradation, matching
  the behaviour of the existing test_graceful_na_when_probes_fail test.

Notes

No new dependencies. The @patch decorator and FileNotFoundError
side-effect pattern are already used extensively in this file.