fix: pick OS-specific ffmpeg binary in hls demux fallback test

`test_demux_flac_uses_tools_dir_fallback` hard-coded `tools_dir / "ffmpeg"`
in its `fake_exists` stub, but `_demux_flac` looks for `ffmpeg.exe` on
Windows (os.name == 'nt'). Result: the fake_exists stub never matched,
the code fell through to the "ffmpeg is required" RuntimeError instead
of the expected "ffmpeg failed" subprocess error, and the test failed
on Windows. Linux CI passed because os.name == 'posix' uses bare
"ffmpeg".

Pick the binary name based on `os.name` to match what `_demux_flac`
actually probes for. Asserts on the matching candidate path.

Tests: 20 passing on Windows (was 19/20). Ruff clean.

tests/test_hls_parsing.py

@@ -7,6 +7,7 @@ The parsing logic is identical in ``tidal_download_client.py``; these tests
 cover the shared behavior via ``hifi_client.HiFiClient``.
 """
 
+import os
 import shutil
 import subprocess
 import sys
@@ -356,12 +357,15 @@ def test_demux_flac_uses_tools_dir_fallback(client, tmp_path, monkeypatch):
     out = tmp_path / "output.flac"
 
     tools_dir = Path(__file__).parent.parent / "tools"
+    # _demux_flac picks the OS-specific binary name (ffmpeg.exe on Windows).
+    candidate_name = "ffmpeg.exe" if os.name == "nt" else "ffmpeg"
+    expected_candidate = tools_dir / candidate_name
 
     original_exists = Path.exists
     original_which = shutil.which
 
     def fake_exists(self):
-        if str(self) == str(tools_dir / "ffmpeg"):
+        if str(self) == str(expected_candidate):
             return True
         return original_exists(self)
 
@@ -381,4 +385,4 @@ def test_demux_flac_uses_tools_dir_fallback(client, tmp_path, monkeypatch):
             client._demux_flac(inp, out)
 
     call_args = mock_run.call_args[0][0]
-    assert call_args[0] == str(tools_dir / "ffmpeg")
+    assert call_args[0] == str(expected_candidate)